a large number of 表示许多的，只能修饰可数名词；a lot of等于lots of，即可修饰可数的，又可修饰不可数名词；plenty of也是修饰可数或不可数名词，与a lot of同义但不如其常用；a great many只能修饰可数名词,等于a good many。记住这些在使用时就不会混淆了。例句：
1、A large number of high-tec products have(has) been made in that factory(那个工厂生产了许多高科技产品）a number of做主语时，动词可用单数，也可用复数。
2、Don't hurry. We still have plenty of time.
There are plenty of such persons in our school.可数和不可数都可以。
3、He got a lot of(lots of)homework(things)to do after school.可数和不可数都可以。
4、They raised a great many(a good many)proposals at the meeting.只能是可数的。
此外，表示大量，许多的还有a large quantity of，修饰可数和不可数名词，如：
As a result of destroying the forests, a large quantity of desert has coverd the land(毁林导致了大量土地的荒漠化）. On that day she received a large quantity of flowers.
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