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.