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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.
