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动词表示动作形态，但英语里有些动词并不表示动作形态，而是协助主动词表示时态（tenses）或语态（voices）。这些动词如 do, is, have等，叫助动词（auxiliary verbs）。另外有些动词如can, will, may, must等，则可协助动词表示语气或情态，叫情态动词（modal verbs）。 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 wont. ● -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. | 
