在句子中,“be”可以是主动词(The Principal Verb)或助动词(The Auxiliary Verb).
1.当主动词时,“be”在性质上属于接系动词(The Linking Verb),(1) 后面要有名词、形容词、地方副词或短语作补足语(The Complement).例如：
The man is a science teacher.
(2)当“be”要在祈使句中出现时,它必须借重助动词“do”或“don't”之助,如：
Don't be silly!
2.“be”作助动词时
(1)Be + 现在分词”以组成进行式时态(Continuous Tenses),如：
Tony's maid is washing his new car.
(2)“Be + 过去分词”以组成被动语态(The Passive Voice),如：
Her money in the drawer was stolen.