什么时候用be,什么时候用is和are呢?

be动词
be动词的用法
be
[bi:]
vi
现在时 I am, you are, he is, we are, you are, they are （缩略式 I'm, you're, he's, we're, you're, they're）, （否定缩略式 I'm not, isn't, aren't）, 过去时 I was, you were, he was, we were, you were, they were （过去时否定缩略式 wasn't, weren't）, 过去分词 been, 现在分词 being

“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 teacher?

6. Are Mary's new dresses colourful?

7. Have I been there before?

8. 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 Park?

23. 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.

你只要记得be是is are的原形，is 是单数 are 是复数

I 后面接 am
we you they后面接 are
he she it后面接 is

一般is are 前面都有带特指的主语，如第三人称he/she/it is，第二人称you are，或所以人称we /you/they are。而be 一般用在will 后面，表将来如he will be a doctor，再看看he is a doctor，对比一下就知道了。还有有听过to be a doctor ，有听过to is a doctor吗。
总归一句话，be 是一个系动词，也叫be 动词，在不同情况下都可以变成is are am was were。不要想复杂了。

is和are在英语里被称为be动词, 在老师讲文法的时候,因为主语不确定会写为be动词形式,is通常加在he, she, it之后，are加在they, us, you后面，也就是前者是单数，后者复数，am用来和I 搭配。
be也有单独的用法， 例如be careful,be thankful to sb. 通常用来提示某种动作或意向。