1. 如果表语是不定式，主语也是不定式；表语是动名词，主语也是动名词。如：
To see is to believe.＝Seeing is believing.
2. 动名词作主语的句型。如：
It is no use/no good/useless doing sth.
It is a waste of time doing(也可用It is a waste of time to do)
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