5、be 动词的祈使句

be 动词的祈使句有肯定和否定两种形式。肯定形式是以be 动词开头，而否定形式或强调句形，要加don't 或do。例如：

Be careful!

Be a good boy!

Don't be silly!

Don't be a fool!

Do be obedient!

Do be careful.

二、be 动词做助动词

助动词be 有两个基本用法，一是与谓语动词一起构成各种时态，二是构成被动语态。

1、“be + 现在分词组”成各种进行式时态。例如：

Tony's maid is washing his new car.

The children are playing in the field.

Samuel was eating when I came in.

We have been living here since 1959.

This time next day they will be sitting in the cinema.

三、注意事项

英语句子中，为了表达时态或被动语态的需要，常常会出现几个助动词连在一块儿使用。也就是说，助动词be会和其它助动词一起使用。这时候，整个句子的一般疑问句和否定句，就不一定是对be 进行变化了，而是以第一个助动词为主进行变化。即哪个助动词在先，就以它为主进行变化。例如：

[肯定句] All the wonderful birds can be found in our Bird Park. (助动词有两个：can be )

[疑问句] Can all the wonderful birds be found in our Bird Park? (移第一个can)

[否定句] All the wonderful birds can not be found in our Bird Park. (在第一个can 后加not)

[肯定句] I have been there before. (两个助动词：have, been )

[疑问句] Have you been there before?

[否定句] I have not been there before.

[肯定句] I shall be having my supper at six tomorrow evening. (两个助动词：shall, be )

[疑问句] Will you be having my supper at six tomorrow evening?

[否定句] I shall not be having my supper at six tomorrow evening.