祈使句用以表达命令，要求，请求，劝告等。

1) 祈使句有两种类型，一种是以动词原形开头，在动词原形之前加do (但只限于省略第二人称主语的句子)。

Take this seat.

Do be careful.

否定结构：

Don’t move.

Don’t be late.

2) 第二种祈使句以let开头。

Let 的反意疑问句

a. Let’s 包括说话者

Let’s have another try，shall we / shan’t we?

= Shall we have another try?

b.Let us 不包括说话者

Let us have another try，will you / won’t you?

= Will you please let us have another try?

否定结构：

Let’s not talk of that matter.

Let us not talk of that matter.