A In my class most students use e-mail
B In my class the most students use e-mail
Both are correct. The second one is perhaps more rare (usually you can find it as "most of the students") but it's certainly grammatical. COBUILD concordance sampler gives plenty of results.
2) I’d like to know what you do for a:Work b: A living c: A job d: A profession
d: profession wrong
It's not a matter of Count or Mass (Uncount) noun, but rather of idiomatic usage.
If I would have been you I would have acted differently
Wrong. Usually you don't use "would" like this, except in sentences such as "If you would please answer, I would be grateful".
Correct would be: "If I were you, I would have acted differently". It's also possible to say: "If I were you, I would act differently" (it indicates generic, habitual use)
If I had been you I would have acted differently.
Not used. You can find it in older texts, such as in Jane Austen, but a search in COBUILD concordance sampler returned no results.