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.