13.1 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.