a number of…
a number of…意思是“一些,若干”（= some）,后接可數(shù)名詞復數(shù)或代詞,謂語動詞用復數(shù)形式.
a number of…詞組中還可以加入形容詞表示數(shù)量大或小等（例如：a large/small number of…許多/少數(shù)……）.
注意和the number of的區(qū)別.the number of…意思是“……的數(shù)字/數(shù)目”,介詞of同其后名詞構(gòu)成介詞短語,修飾the number.當它作主語時,謂語動詞用單數(shù).例如：
Many people took part in 10,000-metre race,but only a number of them kept on running to the end.許多人參加了10000米跑,但只有一些人堅持跑到底.
A number of my friends think I should take a holiday.
我的一些朋友認為我應該休假.
The number of students is about twenty.
學生人數(shù)大約是20人左右.
the number of 后面用單數(shù).