Is the word "much" used in affirmative sentences?
I want to buy much milk.
I know that "much" is used in negative sentences and questions, but I am not sure if using it in affirmative sentences is grammatically correct.
There is more in that question than meets the eye.
Much is correct in the affirmative (a large quantity or amount, see American Heritage). It so happens that it is a) slightly archaic b) not often used with concrete words (water, butter, etc.) or ordinary quantities (money). Hence that would sound a little passé in normal conversation:
I bought much milk.
I bought a lot of milk.
My best guess: much still belongs to an educated register and more likely to be used with set phrases:
I am much obliged.
You will avoid much grief.
He knows much.
Much ado about nothing.
Aside of much + comparative, which also belongs to spoken language:
It's much better.
By contrast a lot is informal and not particularly advisable in written style. One would write:
A large quantity of water
A large amount of butter
A lot did displace much but that leaves an awkward gap in formal language, requiring clunky periphrases. Hence I would argue that:
He bought much milk.
still does have its place in formal style, for brevity. But that would probably be a matter of opinion.