I'm happy to give my 2 cents here.
You propose two sentences:
1: You never can work too carefully.
2: You can never work too carefully.
I will say, to the best of my understanding, neither one of these is outright incorrect. The word "never" would not work later in the sentence --- that would produce grammatical errors. Between these two, the different is more subtle, but both are acceptable. Some folks would say, both are 100% grammatically correct.
Technically, the first says: one "never can", that it, one does not have the ability or the possibility, to work too carefully. Technically, the second says: one positively does have the ability never to work carefully, or to work carefully never. The first, properly, is a quasi-moral admonition, exhorting us all to the ideal of ever-higher standards of care. The second is stranger: with the proper inflection, it could be read as if I practice and cultivate the skill of assiduously avoiding working too carefully; with another inflection, it would imply that while you can't *work* too carefully, there are other things you can do too carefully. In practice no one will say it like that, so in practice, when people hear #2, they will think the idea of #1. Of course, though, one should always strive to say exactly what one means.
If you think about what happens when you put the word "never" at the beginning of the sentence:
3: Never can you work too carefully.
one gets this somewhat poetic rendition of the moral exhortation. Clearly, the word "never" needs to be with the word "can" --- the sentence doesn't work if any word other than "can" is the second word. That's one way to hear why "never" must be next to "can."
One particularly fine point of grammar is to avoid splitting up the verb. Folks who are really picayune about hyperfine grammatical distinctions get worked up about this. If we take that into account, then version #1 is superior to both #2 and #3 because it keeps the verb ("can work") intact and unseparated. Thus would the high grammatical muck-a-mucks decree.
All this is way way beyond what you are going to be expected to do on GMAT SC. Differences between right and wrong answers on SC will not get into subtle issues like this. GMAT SC explores much more gross distinctions. For example:
(A) A person who eats a high fat diet needs to see a doctor.
(B) A person needs to see a doctor who eats a high fat diet.
In this example, (A) is correct, and (B) is a flagrant violation of the modifier-touch rule. Sentence (B) has a trainwreck error. Most of the time on GMAT SC, each wrong answer will have an unambiguous trainwreck error. The distinction between "never can work" and "can never work" it too subtle, too much a matter of taste, to constitute by itself the difference between a right and a wrong answer choice on GMAT SC. If something like "never can work" vs."can never work" is the only different you observe between two choices, chances are good that somewhere else, there's a small but much more decisive difference (e.g. one might have a missing "s" at the end of a verb).
Does all this make sense? Please let me know if you have any questions.
Mike
