Q:

Disable indentation for one paragraph

I have written the following :
\par{first paragraph}
\par{second paragraph}
\par{third paragraph}

Now, I want to disable indentation for the third paragraph.
I naturally add \noindent command as following :
\par{first paragraph}
\par{second paragraph}
\noindent\par{third paragraph}

But it doesn't produce any effect.
Then, I found several ways to achieve that, but I still ask myself why \noindent is not working properly.
Thanks for sharing !

A:

First off, I strongly suggest you undo an apparent writing habit that has a worrisome cargo cult programming flavor, i.e., that you replace
\par{first paragraph}
\par{second paragraph}
\par{third paragraph}

with 
first paragraph

second paragraph

third paragraph

Why? Mostly to reduce pointless code clutter, but also to acknowledge that (a) \par is a switch and does not take an argument and (b) TeX creates paragraph breaks automatically whenever it encounters one or more all-blank lines. 
Second, to inhibit the paragraph indentation which happens if the parameter \parindent takes a positive length, all you need to do is to insert \noindent at the start of the paragraph, or paragraphs, in question. 
Finally, it should be obvious why \noindent\par{third paragraph} doesn't produce the desired effect: \noindent occurs at the end of the second paragraph and not at the start of the third paragraph. Hence, it can have no effect on the indentation of the first line of the third paragraph.

