1. 两者都表示“许多”，但 many 修饰或代替可数（复数），与 few（少数）相对；而 much 用来修饰或代替不可数名词（单数），与little（少量）相对。如：
Did you see many people there? 你在那儿看见许多人了吗?
Many poets have died young. 许多诗人很年轻就死了。
Do you have much money left? 你剩的钱多吗?
He doesn't spend much time preparing his lessons. 他备课不花太多时间。
2. 关于many of 和 much of：
(1) 其后接名词时，该名词通常应是特指的（比如有the, these, those, my, our, Tom's等修饰）。如：
Many of the farmers grow rice. 很多农民种稻子。
Many of his ideas were amusing to her. 他的许多想法使她感到有趣。
He spends much of his time outdoors. 他很多时间都在户外度过。
Much of the land was flooded. 大片土地被水淹没。
误：She didn't eat much of breakfast.
正：She didn't eat much breakfast.
误：There aren't many of large glasses left.
正：There aren't many large glasses left.
误：Many of students have read it.
正：Many of the students have read it. 许多学生读过它。
误：Much of time was wasted.
正：Much of the time was wasted. 大部分时间都浪费了。
(4) 在少数情况下，much of之后不跟限定词也可以用，这主要见后接人名和地名的情形。如：
Not much of Denmark is hilly. 丹麦山地不多。
I've seen too much of Howard recently. 最近我见霍华德见得太多了。
(5) 在人称前总是用many of或much of，其中的of不能没有。如：
I didn't eat much of it. 这个我没吃多少。
Many of us were too tired to go further. 我们很多人都累得不能再往前走了。
(6) much of有时可用于一个还有不定的名词前。如：
You can't see much of a country in a week. 一周之内看一个国家看不了多少地方。
How much of a job would it be to rebuild the garage? 重建这个车库工作量会是多少?
not much of a 有时可用于名词前表示某人对某物或某人评价不高。如：
Then he's not much of a partner. 那时他不是什么了不起的合伙人。
That might not seem like much of an accomplishment. 那似乎算不上什么成就。
3. 两者都可与 more 连用，其区别仍然是 many more之后接复数名词，much more 之后接不可数名词。如：
There have been many more burglaries this year. 今年盗窃案比过去多好些。
His car cost much more money than mine. 他的小汽车所花的钱比我的多得多。
Many people support the government, but many more are against it. 许多人支持政府，但更多得多的人反对它。
另外，其后修饰的不是名词，而是或，则用much more，不用many more。如：
You must work much more carefully. 你应工作得更加细心。
Politics is much more difficult than physics. 政治学比物理学难。
4. 注意“many a+单数可数名词”的用法：形式上为单数，但意义上为复数（意为“许多”），若用作主语，其谓语通常与形式一致而不与意义一致（即谓语用单数）。如：
I have told him many a time to be polite. 我多次告诉他要有礼貌。
Many a young man has tried and failed. 很多年轻人都试过，而且失败了。
注意，“many a+单数名词”是一个十分正式的结构，在口语或非正式文体中一般都用 “many+复数名词”代之。
5. 与a great [good] number of, a great [good] amount of等表达不同，a great [good] many后接名词时，不用介词of。如：
I've known her for a great many years. 我认识她好多年了。
A great many people were at the meeting. 很多人参加了会。
A good many of them have cars. 他们中的许多人有汽车。
A great many of the trees were destroyed in the storm. 这次暴风雨毁了许多树。