语法：祈使句结构

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.