The adverb tends to modify the word it is closest to.
It is difficult to exactly determine what makes businesses succeed or fail.
Here exactly is modifying "to determine."
But the infinitive form ("to determine") is split by the adverb (the adverb has separated "to" from "determine") and this is frequently regarded as an error.
It is difficult to determine exactly what makes businesses succeed or fail.
The adverb is modifying "determine."
The sentence states that an "exactness of determining" is difficult to perform -- that "exact determinations" are hard to do.
However, the intonation of the voice can make "exactly" modify "what," so that the sentence becomes the same as the third one.
"It is difficult to ... determine EXACTLY .. what makes a business succeed or fail."
This makes the word modify "determine"
"It is difficult to determine ...exactly WHAT ... makes a business succeed or fail."
This intonation makes the word modify "what."
It is difficult to determine what exactly makes businesses succeed or fail.
The adverb is modifying "what."
The sentence states that it is hard to pin down the exact element involved in failure -- that the exact cause is hard to find.
It is difficult to determine what makes businesses succeed or fail exactly.
The adverb is modifying "fail."
This sentence doesn't make much sense in writing (and it would be changed in editing to relocate the adverb.) How do you "fail exactly?"
But it would be understood in casual speech to mean "to determine exactly" (or possibly "to determine the exact cause" -- depending on context.)
- The second and third sentences are both fine, and essentially indistinguishable in meaning, especially in written form.
- The first sentence contains a grammatical error (a split infinitive)
- The last sentence has a misplaced modifier.