動(dòng)詞表示動(dòng)作形態(tài)，但英語里有些動(dòng)詞并不表示動(dòng)作形態(tài)，而是協(xié)助主動(dòng)詞表示時(shí)態(tài)（tenses）或語態(tài)（voices）。這些動(dòng)詞如“ do, is, have”等，叫助動(dòng)詞（auxiliary verbs）。另外有些動(dòng)詞如“can, will, may, must”等，則可協(xié)助動(dòng)詞表示語氣或情態(tài)，叫情態(tài)動(dòng)詞（modal verbs）。
助動(dòng)詞和情態(tài)動(dòng)詞都是特殊動(dòng)詞，非常有用。沒有了它們，疑問句不能成立，否定句也難形成。沒有了“have, has, had”完成時(shí)態(tài)（perfect tenses）無法存在；沒有了“be, is, are, am, been, was, were, being”，何來被動(dòng)語態(tài)（the passive voice）？請(qǐng)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，助動(dòng)詞和情態(tài)動(dòng)詞還有下列用途：
㈠構(gòu)成疑問句附加語，如：
● Mr Chen has not become a businessman, has he?
● Jason's lectures are clear and easy-to-understand, aren't they?
㈡構(gòu)成簡(jiǎn)短答語，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構(gòu)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，作為第三人稱現(xiàn)在時(shí)態(tài)（third person present tense）時(shí)，助動(dòng)詞和情態(tài)動(dòng)詞語尾不起變化，也就是不必有“-s”這形式。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”代表其他幾個(gè)助動(dòng)詞：is, am, are, was, were, being, seen）
總而言之，助動(dòng)詞和情態(tài)動(dòng)詞，隨處可見，出現(xiàn)率高，不難使用，也不會(huì)有什么錯(cuò)誤。
助動(dòng)詞和情態(tài)動(dòng)詞都是特殊動(dòng)詞，非常有用。沒有了它們，疑問句不能成立，否定句也難形成。沒有了“have, has, had”完成時(shí)態(tài)（perfect tenses）無法存在；沒有了“be, is, are, am, been, was, were, being”，何來被動(dòng)語態(tài)（the passive voice）？請(qǐng)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，助動(dòng)詞和情態(tài)動(dòng)詞還有下列用途：
㈠構(gòu)成疑問句附加語，如：
● Mr Chen has not become a businessman, has he?
● Jason's lectures are clear and easy-to-understand, aren't they?
㈡構(gòu)成簡(jiǎn)短答語，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構(gòu)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，作為第三人稱現(xiàn)在時(shí)態(tài)（third person present tense）時(shí)，助動(dòng)詞和情態(tài)動(dòng)詞語尾不起變化，也就是不必有“-s”這形式。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”代表其他幾個(gè)助動(dòng)詞：is, am, are, was, were, being, seen）
總而言之，助動(dòng)詞和情態(tài)動(dòng)詞，隨處可見，出現(xiàn)率高，不難使用，也不會(huì)有什么錯(cuò)誤。