祈使句用以表達命令，要求，請求，勸告等。
1)祈使句有兩種類型，一種是以動詞原形開頭，在動詞原形之前加do (但只限于省略第二人稱主語的句子)。
Take this seat.
Do be careful.
否定結構：
Don't move.
Don't be late.
2)第二種祈使句以let開頭。
Let 的反意疑問句
a. Let's 包括說話者
Let's have another try，shall we / shan't we?
= Shall we have another try?
b. Let us 不包括說話者
Let us have another try，will you / won't you?
= Will you please let us have another try?
否定結構：
Let's not talk of that matter.
Let us not talk of that matter.