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不定式作主语，往往用it作形式主语，真正的主语不定式放至句子的后面。
例如：
It's so nice to hear your voice.
听到你的声音真高兴。
It's necessary for you to lock the car when you do not use it.
不用车的时候，锁车是有必要的。
It's very kind of you to help us.
他帮助我们，他真好。
It seemed selfish of him not to give them anything.
他不给他们任何东西，这显得太自私了。
但是，用不定式作主语的句子中还有一个不定式作表语时，不能用It is… to…的句型。另外，这样的句子，不能用动名词作表语。例如：
（对）To teach is to learn.
（错）It is to learn to teach.
（错）To teach is learning.
（错）Teaching is to learn.
典型例题：
The chair looks rather hard, but in fact it is very comfortable to ___.
A. sit
B. sit on
C. be seat
D. be sat on
答案：B. 如果不定式为不及物动词，其后应有必要的介词。当动词与介词连用时，常位于"形容词+动词不定式"结构的末尾。
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