不定式可以做谓语以外的任何句子成分,当然可以做表语。
To see is to believe.
但是要注意一个问题:若主语部分含有实意动词do的任何形式时,做表语的不定式通常省掉不定式符号to。
What you should do is trust me when you see my answer.
I have no choice. What I have to choose is to wait.
补充一个例句:
The only thing you have to do is press the button.
对“20分悬赏要的就来”其他回答
What he wanted to do(他想做),但是还没有做。没有做的情况用to do形式。把What he wanted to do看成句子的主语,后面加谓语WAS。在跟TO DO形式表还没做,TO 可省略

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