英语BE动词什么时候用IS

be是一个连系动词，它有自己不同的形式 现在时：be 进行时：being 过去时：was/were 过去分词：been 它的后面必须跟形容词或名词作表语，与之构成系表结构，充当句子的谓语。

i am a student主语：i 谓语：am a student（系表结构） being是它的现在进行时态，一般不单独用，而是用于现在进行时的被动语态。（being+动词过去分词） the bike is being repaired.单车正在被修理中。 it is being finished by him这正在被他完成中。 been是它的过去分词，也不一般单独使用，而是用于现在完成时、过去完成时以及需要它的句子。 i have been here for a long time.我已经到这里有一段时间了。 至于你的问题，这与你想表达的意思有关。 你的工作是什么？ what do you dowhat is your job那么这里选用do和is有什么根据呢？ 第一句里面，第二个do是指你做什么，也就是你的工作，就是一个实义动词。按中文说what you do已经足够了。可是英语里面就不行，必须跟一个助动词，也就是do. 这里就有了一个结论：当句子中有动词时，我们就用助动词do或是does.如果没有动词时，我们就选用is或是are，也就是b“Be”除了原形的“be”之外，还有另外七种形式：am, is, are, been, being, was, were. 在句子中，“be”可以是主动词(The Principal Verb)或助动词(The Auxiliary Verb). 当主动词时，“be”在性质上属于接系动词(The Linking Verb), 后面要有名词、形容词、地方副词或短语作补足语(The Complement)。例如： 1. The man is a science teacher. 2. Mary's new dresses are colourful. 3. I have been there before. 4. Mother is in the kitchen now. 这四个都是陈述句，可以变成疑问句，方法简单，把主语和“be”或助动词对换位置即可： 5. Is the man a science teacher6. Are Mary's new dresses colourful7. Have I been there before8. Is mother in the kitchen now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： 9. Don't be silly! 10. Do be obedient! 11. Don't be a fool! “Be”有两种缩写法，如下： 12. He's not...../He isn't.... 13. You're not...../You aren't... 但“am + not”的缩写法只有一个： 14. I'm not. 有人用“ain't”, 但这并不是标准英语。 谈过了“be”作主动词的功能，现在看看“be”作助动词时，有些什么用法： 1.“Be + 现在分词”以组成进行式时态(Continuous Tenses)，如： 15. Tony's maid is washing his new car. 16. The children are playing in the field. 17. Samuel was eating when I came in. 18. We have been living here since 1959. 2.“Be + 过去分词”以组成被动语态(The Passive Voice),如： 19. Her money in the drawer was stolen. 20. A number of good jobs are taken up byforeigners. 21. David studied in Taiwan but has been trained as a language lecturer in America. 22. Can all the wonderful birds be found in our Bird Park23. The disobedient boy was told to stand out- side the classroom. 24. Steps are being taken to reduce traffic con- gestion during peak hours. 25. Dishonest students will be immediately dealt with. 当陈述句的谓语是实义动词时，助动词do通常用于疑问句和否定句中。如：I go to school every day.我每天上学。Do you go to school every day 你每天都上学吗？I don”t go to school every day.我不是每天都上学。另外助动词do还可用于强调句中，用于强调谓语动词。如：将助动词do及do 的相应形式放于动词前，而动词要用原形。又如：I did tell him.我的确告诉他了。He does like playing football.他的确喜欢踢球。其余时候do作动词原形，表示做.