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.