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 (但只限於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.