many, much与a lot of三者都表示“许多”。
many 和much即可用于肯定句中， 也可用于否定句和疑问句中
many只修饰可数名词复数，常构成how many, too many搭配
People cut down many trees, so animals are in great danger.
人类砍伐了大量的树木，导致动物处于极大的危险之中。
much只修饰不可数名词,常构成how much, too much, much too搭配
There is not much water in the bottle.这个瓶子里没有太多水了。
Thank you for telling me so much knowledge about nature.谢谢你告诉我这么多关于自然的知识。
a lot of(＝lots of)既可以修饰可数名词复数，又可以修饰不可数名词,但是只能用于肯定句中。如果要将含有a lot of(lots of)的肯定句改为否定句或疑问句，则必须用many或much代替。
There are a lot of students in the library.
否定句：There are not many students in the library.
