13.1 祈使句结构
?
  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.













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