Q:

Franz Kafka translation

How do you say the following adverbs in German?

Unfathomably
Senselessly
Terribly 

My understanding is that one must add the suffix "lich" to make a word like unfathomable or senseless an adverb (unergründlich or sinnloslich).
I am trying to translate a quote from Franz Kafkas letters to Milena to its original German but I don't know if I have the right translation.

A:

In fact, most of the German adverbs are not built according to the rule you state. unergründlich is an adjective and already has the "adverb suffix" you state. In fact, a lot of German adjectives do. German is by far not as strict with adverbial use of adjectives as English, that strictly asks for a -ly suffix in adverbial use - In German, in a lot, if not most of places, you simply use the adjective unchanged. 

Susan is careful.
Susan drives carefully.

while in German:

Susanne ist vorsichtig.
Susanne fährt vorsichtig.

You can see that vorsichtig is still an adjective in this adverbial usage because you can form comparative and superlative sentences like

Susanne fährt vorsichtiger als Hans.

(The main adverb/adjective distinction in German is: A pure adverb cannot form comparative or superlative).
A pure adverb is built from the adjective above like

Susanne fährt vorsichtigerweise langsam um die Kurve. 

So - no "-lich" at all.
Note the build rule for vorsichtigerweise is by far not as common as adding the -ly suffix in English. It does somehow work for your examples, though:

Unergründlicherweise hat mir der Mann sein ganzes Geld gegeben. (unfathomably)
Sinnloserweise hat er die ganze Mühe auf sich genommen. (senselessly)

but

Das hat er aber sinnlos aufwendig ausgeführt (senselessly elaborate)
Schrecklicherweise sind alle Insassen ums Leben gekommen. (terribly)

but 

Die Verunglückten wurden schrecklich verstümmelt (terribly as well)

