一、a number of ,the number of
a number of 意思是“许多”，相当于a lot of ; the number of意思是“……的数目,……的数量”，当它作主语时，谓语动词用单数形式。
A number of students like playing computer games.
许多学生喜欢玩电脑游戏。
The number of the students is about 1500 in our school.
我们学校学生的人数大约是1500。
二、基数词变序数词助记歌。
基变序，有规律，末尾加上th;一、二、三特殊记，八加h ，九去e ，ve要用f替;以e结尾变ie，后跟th莫忘记。要想表示几十几，只变个位就可以。
特殊变化：one—first ,two—second ,three—third ,five—fifth ,
nine—ninth ,twelve—twelfth ,forty—fortieth ,
eight-nine—eighty-ninth
三、概数(略数)表达法
数词+hundred/thousand/million/billion+名词复数
hundreds/thousands/millions/billions of+名词复数
例1
1. __people go swimming in summer in Dalian.
A、Thousands B、Thousand of C、Thousands of D、Thousand
2. __the students will take part in this English speech contest.
A. Two hundred of B. Two hundreds of C. Hundreds of D. Hundred of