a lot of 是既可以修饰可数名词,也可以修饰不可数名词,
many只能修饰可数名词的复数,
还有,a lot of 可以修饰比较级,而many不可以.
a lot of 可以用以简略回答,而many不可以
例如:What have you learnt for this week?
I have learnt a lot.