助动词和情态动词都是特殊动词，非常有用。没有了它们，疑问句不能成立，否定句也难形成。没有了“have, has, had”完成时态（perfect tenses）无法存在；没有了“be, is, are, am, been, was, were, being”，何来被动语态（the passive voice）？请看下列例子，就知道梗概了：
● -What do you do every day?
-I teach every day.
● -Can they speak English?
-Yes, they can.
● -Will he do it?
-No, he won't.
● -John lived here before.
-John did not live here before.
● -Betty has completed her work on time.
-She is really good.
● -After I had eaten my breakfast, I went to office.
-How about Nancy? Did she do the same?
● -Jason is writing a new book.
-A new book is being written by Sam, too.
● -The cat caught a rat.
-A rat was caught by a cat.
此外，助动词和情态动词还有下列用途：
㈠构成疑问句附加语，如：
● Mr Chen has not become a businessman, has he?
● Jason's lectures are clear and easy-to-understand, aren't they?
㈡构成简短答语，如：
● -Who can answer this question?
-Sam can.
● -Will you help us with the gardening?
Yes, I will. /No, I won't.
● -I have seen that film before.
-So have I.
● -William isn't a hypocrite.
-Neither is his wife.
㈢构成省略句，如：
● I can't understand him. Can you ?
● The boss doesn't often come to the factory and when he does, it is generally on Sunday.
除了“do, be, have”之外，作为第三人称现在时态（third person present tense）时，助动词和情态动词语尾不起变化，也就是不必有“-s”这形式。例如：
I can write English. He can write English, too.
Tom will know soon, but he will never know.
All of you may go. Jack may also go if he wants to.
（注意“be”代表其他几个助动词：is, am, are, was, were, being, seen）
总而言之，助动词和情态动词，随处可见，出现率高，不难使用，也不会有什么错误。
