2、在句子中，be有兩種主要作用：一是作為系動詞(The Linking Verb)，本身可以作用謂語動詞；二是做為助動詞(The Auxiliary Verb)，與謂語動詞一起構(gòu)成各種時態(tài)和被動語態(tài)等。

　　下面做個簡要的講解。

　　一、be 動詞做系動詞

　　1、系動詞+表語”的結(jié)構(gòu)

　　當(dāng)be 動詞做系動詞使用時，主要構(gòu)成“系動詞+表語”的結(jié)構(gòu)，在句子中做謂語。表語可以是名詞、形容詞、副詞、介詞短語等。例如：

　　The man is a science teacher.

　　Mary's new dresses are colourful.

　　Mother is in the kitchen now.

　　I have been there before.

　　They shall be here at 10:00 tommorow.

　　That can't be true.

　　You are not being very polite.

　　Your brother is being very annoying this evening

　　2、be 動詞的一般疑問句，方法是把be 移到主語前面，也可說成是移到句首。

　　Is the man a science teacher? Yes, he is. / No, he isn't.

　　Are Mary's new dresses colourful? Yes, they are. / No, they aren't.

　　Was mother in the kitchen then? Yes, she was. / No, she wasn't.

　　Were you at home the day before yesterday?  Yes, I was. / No, I wasn't.

　　Was she late this morning? Yes, she was. / No, she wasn't.
 

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