user157: In most of the translations of the Dhammapada I have read the first lines of the Twin Verses (first chapter) are rendered using the words mind or thoughts. For instance this translation by Buddharakkhita 

Mind precedes all mental states. Mind is their chief; they are all
  mind-wrought.

However I recently read a translation by Thanissaro (in this book) that renders the lines as

Phenomena are preceded by heart, ruled by heart, made of heart

I found the variation from mind (or thoughts) to heart quite striking. What is the original Pali phrasing and which words are able to be translated freely between mind and heart? What is the overall sense of the Pali words used i.e. what are the nuances or subtleties to the original Pali?

user254: Here is what I found:

Manopubbangama dhamma
  manosettha manomaya
  manasa ce padutthena

There's translation and commentary on the page above.
The associated story ("The Story of Thera Cakkhupala") suggests it might be 'intention'.
That's kind of compatible with the poetic use of heart: "he had a change of heart", "his heart wasn't in it", "he did it with all his heart", etc.
The root syllable in each line is "mano" and/or "mana": which can also be found here associated with "mental consciousness"; but see also this says that it's used for "mental action" i.e. kamma.
My guess is that:

Pali is constructed using short syllables like 'man'
The syllables have an approximate meaning
The suffix changes depending on grammar
The syllables are aggregated into larger compound words and/or used with other words in a sentence
There may be a basic or root meaning (perhaps from the root's etymology) but the specific/precise  meaning (of a syllable) depends on context
The original specific meaning is non-trivial for Pali scholars to render using single English words (for example the several words used to translate dukkha)
You might find it useful to know at least the approximate/root meaning
You might also find it useful to know that the meaning is a bit approximate!

An example of one word in English which has many specific meanings is, just to give an example, "run": you can run a race, run a horse, run an engine, run a bath, run a business, let things run their course, run away, run the printing presses, run for President, etc. as described in a dictionary.

user-1: "What is the original Pali phrasing?"
Verse I

Manopubbangamā dhammā manosetthā manomayā,
manasā che padutthena bhāsati vā karoti vā
tato nan dukkha manveti chakkan va vahato padan.

Verse II

Manopubbangamā dhammā manosetthā manomayā,
manasā che pasannena bhāsati vā karoti vā
tato nan sukhamanveti chāyā va anapāyinī.

"and which words are able to be translated freely between mind and heart?"
I'm not sure (see @yuttadhammo's answer), but Thanissaro is using "heart" to translate manas.
"What is the overall sense of the Pali words used i.e. what are the nuances or subtleties to the original Pali?"
Sue Hamilton [1] discusses these passages:

In the Dhammapada however, both terms [manas and dhammā] are being used in their generic sense [not as sense and sense object, as elsewhere in the pali canon]: manas is being used as 'mind' in general, and dhammā is being used as it is in the tilakkhana [2] formula [...].
When these terms are used generically, the precise meaning of the passage has to be determined from context. The [first line of both verses] literally means something like: "Phenomena are the result of mind (or are preceeded by mind), have mind as their best, are mind-made". In isolation, this sentence might be construed as positing an idealistic ontology, that the phenomena which comprise the world as we know it, dhammā, are nothing but the mind: the external world is magically created by the mind and consist of mind. There is an alternative way of interpreting this line, however, which is, in my opinion, the accurate one.

Then she offers a full translation of the rest of the passage. Excluding the first lines:

"If a man speaks or acts with a wicked mind, sorrow follows him as wheel follows the foot of a draught [animal]...if a man speaks or acts with a pure mind, happiness follows him as a shadow always follows [him]."

Then she points out that:

The point of these sentences is that one reaps the consequences of one's actions [...]. This corresponds to the Buddha's definition of karma [volition] [...]. The whole of the first chapter in Dhammapada is concerned with this teaching: that one reaps as one sows, and that sowing is qualitatively determined by intention.

With the above in mind, and in the same vein as @ChrisW answer , "heart" could've been a choice for someone's inclination. Perhaps to avoid giving the impression of an ontological statement.
Finally:

I mentioned in chapter IV that when used in a non-technical sense, 'mind' (whatever term is being used) indicates general mental activity. So the first lines of the two verses in fact have the following meaning:
dhammā are an individual's experiences -- everything, in fact, that is a part of the individual's life. And qualitatively those experiences follow from one's mental activity: manopubbangamā, it is mental activity which principally governs the nature of life: manosetthā, and it is in mental activity that what follows originates: manomaya.
To interpret this sentence ontologically is completely to ignore the context in which it is found and to divorce it from the subject matter of the entire chapter. To convey the meaning of manas in this context accurately, it is better to translate it as a verbal noun, denoting the activity or process of the mind: 'thinking'. In english, this gives us a less ambiguous sentence:
"The individual's experiences are preceded by thinking, have thinking as their best, originate in thinking".
It is the reification of manas as 'mind' which tends to mislead here


[1] Identity and Experience: The Constitution of the Human Being According to Early Buddhism, pg 140ff
[2] The Three marks of existence: "sabbe saṅkhāra aniccā. sabbe saṅkhāra dukkhā. sabbe dhammā anattā"

user18: Thanissaro's translations are... unique. The word being used, as others have pointed out, is "mano", the leader of an irregular group of nouns called the "manogana" - "the group with mano as its leader".
The PED has a huge article on mano, worth reading in full if you're interested. I've copied the gist of it below, but as you should see, it is probably misleading to translate it as "heart". There is a word for that, "hadaya", which is sometimes used as a synonym of mano, viññāṇa, etc., but also refers to the physical heart organ.
It is fair to say he is within his right to use poetic license in this case; it is, after all, poetry, and the mind and the heart are much the same in Buddhism. As you can see below, though, manas generally refers more to the thinking side of things, than the emotive or the substantial entity of "the heart" or "the mind".

Mano & Mana(s); (nt.) [Vedic manaḥ, see etym. under maññati]

...

II. Meaning: mind, thought 

...


Mano represents the intellectual functioning of consciousness, while viñnāṇa represents the field of sense and sense -- reaction ("perception"), and citta the subjective aspect of consciousness (cp. Mrs. Rh. D. Buddhist Psychology p. 19) -- The rendering with "mind" covers most of the connotation; sometimes it may be translated "thought. As "mind" it embodies the rational faculty of man which, as the subjective side in our relation to the objective world, may be regarded as a special sense, acting on the world, a sense adapted to the rationality (reasonableness, dhamma) of the phenomena, as our eye is adapted to the visibility of the latter.


...


As regards the relation of manas to citta, it may be stated that citta is more substantial (as indicated by translation "heart"), more elemental as the seat of emotion, whereas manas is the finer element, a subtler feeling or thinking as such. See also citta2 I., and on rel. to viññāṇa & citta see citta;2 IV. 2b. In the more popular opinion and general phraseology however manas is almost synonymous with citta as opposed to body, cittaŋ iti pi mano iti pi S ii.94. 



As to your ancillary questions:
What is the original Pali phrasing?
As others have pointed out, the original is:

manopubbaṅgamā dhammā, manoseṭṭhā manomayā.
  manasā ce paduṭṭhena, bhāsati vā karoti vā.
  tato naṃ dukkhamanveti, cakkaṃva vahato padaṃ.


Which words are able to be translated freely between mind and heart?
Some words used more or less interchangeably are citta, viññāṇa, mano, hadaya, ceto and nāma. Any one of these could ostensibly be translated as either "heart" or "mind". The only one that literally means "heart", as mentioned, is hadaya. The rest have colloquial connotations and are derived from various roots, e.g. √cit = to think, √ñā = to know, √man = to consider, or not from roots at all (hadaya = heart, nāma = mentality).

What is the overall sense of the Pali words used i.e. what are the nuances or subtleties to the original Pali?
The technical meaning of the verse is that wholesome and unwholesome deeds come from wholesome or unwholesome mind states, not mere physical acts. In this, Thanissaro's translation is misleading. The emphasis should be on the fact that it is the mind as opposed to the body, not the heart as opposed to the mind.
The origin story about the elder Cakkhupala makes this clear; during his walking meditation, many insects were killed because he was blind. The Buddha spoke this verse to show that Cakkhupala was innocent.
To get an understanding of the colloquial meaning of this verse is a bit involved, but reading the PED's article on how mano is generally used might help. Its usage here does seem to be to emphasise the mental nature of guilt and innocence. Otherwise, citta is a more common word. The word for heart, hadaya, is used when talking about emotion, e.g. hadayaṅgamā - going to one's heart (i.e. heartwarming), and doesn't seem appropriate in this context.

157: <p>In most of the translations of the Dhammapada I have read the first lines of the Twin Verses (first chapter) are rendered using the words mind or thoughts. For instance <a href="http://www.accesstoinsight.org/tipitaka/kn/dhp/dhp.01.budd.html" rel="noreferrer">this translation</a> by Buddharakkhita </p>

<blockquote>
  <p>Mind precedes all mental states. Mind is their chief; they are all
  mind-wrought.</p>
</blockquote>

<p>However I recently read a translation by Thanissaro (<a href="http://www.lulu.com/gb/en/shop/lokabandhu/puja-readings-and-other-texts-as-used-in-the-triratna-buddhist-community/hardcover/product-21435684.html" rel="noreferrer">in this book</a>) that renders the lines as</p>

<blockquote>
  <p>Phenomena are preceded by heart, ruled by heart, made of heart</p>
</blockquote>

<p>I found the variation from mind (or thoughts) to heart quite striking. What is the original Pali phrasing and which words are able to be translated freely between mind and heart? What is the overall sense of the Pali words used i.e. what are the nuances or subtleties to the original Pali?</p>
 254: <p><a href="http://www.tipitaka.net/tipitaka/dhp/verseload.php?verse=001" rel="nofollow">Here</a> is what I found:</p>

<blockquote>
  <p>Manopubbangama dhamma<br>
  manosettha manomaya<br>
  manasa ce padutthena</p>
</blockquote>

<p>There's translation and commentary on the page above.</p>

<p>The associated story ("The Story of Thera Cakkhupala") suggests it might be 'intention'.</p>

<p>That's kind of compatible with the poetic use of heart: "he had a change of heart", "his heart wasn't in it", "he did it with all his heart", etc.</p>

<p>The root syllable in each line is "mano" and/or "mana": which can also be found <a href="http://en.wikipedia.org/wiki/Skandha#Eighteen_Dh.C4.81tus" rel="nofollow">here</a> associated with "mental consciousness"; but see also <a href="http://en.wikipedia.org/wiki/Manas-vijnana#Overlapping_Pali_terms_for_.22mind.22" rel="nofollow">this</a> says that it's used for "mental action" i.e. kamma.</p>

<p>My guess is that:</p>

<ul>
<li>Pali is constructed using short syllables like 'man'</li>
<li>The syllables have an approximate meaning</li>
<li>The suffix changes depending on grammar</li>
<li>The syllables are aggregated into larger compound words and/or used with other words in a sentence</li>
<li>There may be a basic or root meaning (perhaps from the root's etymology) but the specific/precise  meaning (of a syllable) depends on context</li>
<li>The original specific meaning is non-trivial for Pali scholars to render using single English words (for example the several words used to translate <em>dukkha</em>)</li>
<li>You might find it useful to know at least the approximate/root meaning</li>
<li>You might also find it useful to know that the meaning is a bit approximate!</li>
</ul>

<p>An example of one word in English which has many specific meanings is, just to give an example, "run": you can run a race, run a horse, run an engine, run a bath, run a business, let things run their course, run away, run the printing presses, run for President, etc. as described in a dictionary.</p>
 -1: <p><em>&quot;<strong>What is the original Pali phrasing?</strong>&quot;</em></p>
<p>Verse I</p>
<blockquote>
<p>Manopubbangamā dhammā manosetthā manomayā,<br />
manasā che padutthena bhāsati vā karoti vā<br />
tato nan dukkha manveti chakkan va vahato padan.</p>
</blockquote>
<p>Verse II</p>
<blockquote>
<p>Manopubbangamā dhammā manosetthā manomayā,<br />
manasā che pasannena bhāsati vā karoti vā<br />
tato nan sukhamanveti chāyā va anapāyinī.</p>
</blockquote>
<p><em>&quot;<strong>and which words are able to be translated freely between mind and heart?</strong>&quot;</em></p>
<p>I'm not sure (see <a href="https://buddhism.stackexchange.com/a/8906/382">@yuttadhammo's answer</a>), but Thanissaro is using &quot;heart&quot; to translate <a href="http://en.wikipedia.org/wiki/Manas_(early_Buddhism)" rel="nofollow noreferrer"><em>manas</em></a>.</p>
<p><em>&quot;<strong>What is the overall sense of the Pali words used i.e. what are the nuances or subtleties to the original Pali?</strong>&quot;</em></p>
<p>Sue Hamilton [1] discusses these passages:</p>
<blockquote>
<p>In the Dhammapada however, both terms [<em>manas</em> and <em>dhammā</em>] are being used in their generic sense [not as sense and sense object, as elsewhere in the pali canon]: <em>manas</em> is being used as 'mind' in general, and <em>dhammā</em> is being used as it is in the <em>tilakkhana</em> [2] formula [...].</p>
<p>When these terms are used generically, the precise meaning of the passage has to be determined from context. The [first line of both verses] literally means something like: &quot;Phenomena are the result of mind (or are preceeded by mind), have mind as their best, are mind-made&quot;. In isolation, this sentence might be construed as positing an idealistic ontology, that the phenomena which comprise the world as we know it, <em>dhammā</em>, are nothing but the mind: the external world is magically created by the mind and consist of mind. There is an alternative way of interpreting this line, however, which is, in my opinion, the accurate one.</p>
</blockquote>
<p>Then she offers a full translation of the rest of the passage. Excluding the first lines:</p>
<blockquote>
<p>&quot;If a man speaks or acts with a wicked mind, sorrow follows him as wheel follows the foot of a draught [animal]...if a man speaks or acts with a pure mind, happiness follows him as a shadow always follows [him].&quot;</p>
</blockquote>
<p>Then she points out that:</p>
<blockquote>
<p>The point of these sentences is that one reaps the consequences of one's actions [...]. This corresponds to the Buddha's definition of karma [volition] [...]. The whole of the first chapter in Dhammapada is concerned with this teaching: that one reaps as one sows, and that sowing is qualitatively determined by intention.</p>
</blockquote>
<p>With the above in mind, and in the same vein as <a href="https://buddhism.stackexchange.com/a/8898/382">@ChrisW answer</a> , &quot;heart&quot; could've been a choice for someone's inclination. Perhaps to avoid giving the impression of an ontological statement.</p>
<p>Finally:</p>
<blockquote>
<p>I mentioned in chapter IV that when used in a non-technical sense, 'mind' (whatever term is being used) indicates general mental activity. So the first lines of the two verses in fact have the following meaning:</p>
<p><em>dhammā</em> are an individual's experiences -- everything, in fact, that is a part of the individual's life. And qualitatively those experiences follow from one's mental activity: <em>manopubbangamā</em>, it is mental activity which principally governs the nature of life: <em>manosetthā</em>, and it is in mental activity that what follows originates: <em>manomaya</em>.</p>
<p>To interpret this sentence ontologically is completely to ignore the context in which it is found and to divorce it from the subject matter of the entire chapter. To convey the meaning of <em>manas</em> in this context accurately, it is better to translate it as a verbal noun, denoting the activity or process of the mind: 'thinking'. In english, this gives us a less ambiguous sentence:</p>
<p>&quot;The individual's experiences are preceded by thinking, have thinking as their best, originate in thinking&quot;.</p>
<p>It is the reification of manas as 'mind' which tends to mislead here</p>
</blockquote>
<hr />
<p>[1] Identity and Experience: The Constitution of the Human Being According to Early Buddhism, pg 140ff</p>
<p>[2] <a href="http://en.wikipedia.org/wiki/Three_marks_of_existence" rel="nofollow noreferrer">The Three marks of existence</a>: &quot;<em>sabbe saṅkhāra aniccā. sabbe saṅkhāra dukkhā. sabbe dhammā anattā</em>&quot;</p>
 18: <p>Thanissaro's translations are... unique. The word being used, as others have pointed out, is "mano", the leader of an irregular group of nouns called the "manogana" - "the group with mano as its leader".</p>

<p>The PED has <a href="http://dsalsrv02.uchicago.edu/cgi-bin/philologic/getobject.pl?c.2:1:3860.pali" rel="nofollow">a huge article on mano</a>, worth reading in full if you're interested. I've copied the gist of it below, but as you should see, it is probably misleading to translate it as "heart". There is a word for that, "hadaya", which is sometimes used as a synonym of mano, viññāṇa, etc., but also refers to the physical heart organ.</p>

<p>It is fair to say he is within his right to use poetic license in this case; it is, after all, poetry, and the mind and the heart are much the same in Buddhism. As you can see below, though, manas generally refers more to the thinking side of things, than the emotive or the substantial entity of "the heart" or "the mind".</p>

<blockquote>
  <p>Mano &amp; Mana(s); (nt.) [Vedic manaḥ, see etym. under maññati]</p>
</blockquote>

<p>...</p>

<blockquote>
  <p>II. Meaning: mind, thought </p>
</blockquote>

<p>...</p>

<blockquote>
  <ol>
  <li>Mano represents the intellectual functioning of consciousness, while viñnāṇa represents the field of sense and sense -- reaction ("perception"), and citta the subjective aspect of consciousness (cp. Mrs. Rh. D. Buddhist Psychology p. 19) -- The rendering with "mind" covers most of the connotation; sometimes it may be translated "thought. As "mind" it embodies the rational faculty of man which, as the subjective side in our relation to the objective world, may be regarded as a special sense, acting on the world, a sense adapted to the rationality (reasonableness, dhamma) of the phenomena, as our eye is adapted to the visibility of the latter.</li>
  </ol>
</blockquote>

<p>...</p>

<blockquote>
  <ol start="3">
  <li><strong>As regards the relation of manas to citta, it may be stated that citta is more substantial (as indicated by translation "heart")</strong>, more elemental as the seat of emotion, whereas manas is the finer element, a subtler feeling or thinking as such. See also citta2 I., and on rel. to viññāṇa &amp; citta see citta;2 IV. 2b. In the more popular opinion and general phraseology however manas is almost synonymous with citta as opposed to body, cittaŋ iti pi mano iti pi S ii.94. </li>
  </ol>
</blockquote>

<hr>

<p>As to your ancillary questions:</p>

<h2>What is the original Pali phrasing?</h2>

<p>As others have pointed out, the original is:</p>

<blockquote>
  <p><strong>mano</strong>pubbaṅgamā dhammā, <strong>mano</strong>seṭṭhā <strong>mano</strong>mayā.
  <br/><strong>manasā</strong> ce paduṭṭhena, bhāsati vā karoti vā.
  <br/>tato naṃ dukkhamanveti, cakkaṃva vahato padaṃ.</p>
</blockquote>

<hr>

<h2>Which words are able to be translated freely between mind and heart?</h2>

<p>Some words used more or less interchangeably are <em>citta</em>, <em>viññāṇa</em>, <em>mano</em>, <em>hadaya</em>, <em>ceto</em> and <em>nāma</em>. Any one of these could ostensibly be translated as either "heart" or "mind". The only one that literally means "heart", as mentioned, is <em>hadaya</em>. The rest have colloquial connotations and are derived from various roots, e.g. √cit = to think, √ñā = to know, √man = to consider, or not from roots at all (hadaya = heart, nāma = mentality).</p>

<hr>

<h2>What is the overall sense of the Pali words used i.e. what are the nuances or subtleties to the original Pali?</h2>

<p>The technical meaning of the verse is that wholesome and unwholesome deeds come from wholesome or unwholesome mind states, not mere physical acts. In this, Thanissaro's translation is misleading. The emphasis should be on the fact that it is the mind as opposed to the body, not the heart as opposed to the mind.</p>

<p>The origin story about the elder Cakkhupala makes this clear; during his walking meditation, many insects were killed because he was blind. The Buddha spoke this verse to show that Cakkhupala was innocent.</p>

<p>To get an understanding of the colloquial meaning of this verse is a bit involved, but reading the PED's article on how <em>mano</em> is generally used might help. Its usage here does seem to be to emphasise the mental nature of guilt and innocence. Otherwise, <em>citta</em> is a more common word. The word for heart, <em>hadaya</em>, is used when talking about emotion, e.g. <em>hadayaṅgamā</em> - going to one's heart (i.e. heartwarming), and doesn't seem appropriate in this context.</p>
