祈使句的结构是：Do sth.（否定式：Don't do sth.）
这个结构是属于句子的范畴。
eg. Keep quiet!
Stop talking!
Don't talk with your mouth full!
Don't put your book here!
不定式的结构是：to do sth，这个结构可以充当主语，宾语，宾语补足语，定语，状语等等。
这个结构只是充当句子的成分。
eg. To see is to believe. （做主语）
I find it useful to learn English well.（不定式做宾语）
He asked me to talk about English study. （不定式做宾语补足语）
I have nothing to say on this question.（不定式做定语）
He stopped to have a rest.（不定式做状语）
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