My friend just told me that his teacher, a native speaker, taught him – that many and much are mainly used in negative sentences and not commonly in affirmative sentences; furthermore, I happened to read an article concerning that matter while searching about it. Here are the examples the article gives:
There aren’t many chairs in the room.
I don’t have much time.
He doesn’t need much money.
As this is not familiar with me, I am rather confused about this. I have never thought that many is not common in affrimative sentences, and because of that I got a question: what adjective should I use then in formal affirmative sentences? I know that a lot of is informal and so is not preferable to use it in formal contexts. For example, what should replace many in the following sentence? And is many that not suitable to be used in affirmative sentences?
He has many questions I could not possibly answer.
I think your friend forgot to mention that his teacher was probably referring to much and many VERSUS declarative sentence with LOTS OF and a lot of. This is what is missing from your context. I am guessing this was what your friend’s teacher meant.
Consider these utterances:
1) I have lots of money but don’t have much free time to spend it.
2) They have a lot of problems but don’t have much time in which to solve them.
3) We have lots of time but not many people to help us with this task.
We would ****not tend to say****:
We have ****much money**** but don’t have much free time to spend it.
We have ****much time**** but not many people to help us with this task.
In sentence two, we would use a lot of/lots of/many rather interchangeably. Nevertheless, in speaking, often, a lot of/lots of is used instead of many for countable nouns.
So the general idea is this: in declarative sentences with uncountable nouns, we tend to use lots/a lot of and a lot of and lots of is also used very often with countable ones.
And the examples show that that much or many are used in the negative idea for the countable or uncountable noun in the same sentence. Interrogative forms are similar to this: Do you have many cousins? Does your brother have much money? Turn those into declarative and you get: I have a lot of cousins (though many is also used, as it is countable). Your brother has a lot of money (much would be very unusual here).
This is probably what your friend’s teacher was trying to say.