祈使句即表示要求或者命令别人做某事或者不要做某事，常用的有肯定句和否定句两种，把祈使句改为否定句只需在动词前加don't即可。
肯定式：以动词原形开头(有时有please)
例句：
Be careful!
Please open your books.
Let me have a try.
否定式：don't+动词原形开头(有时有please)
例句：
Don’t open the door.
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