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.
Don't be late.
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?
Let's not talk of that matter.
Let us not talk of that matter.