动词不定式作主语时，句子的谓语动词常用单数，其位置有以下两种：
1.把不定式置于句首
如：To lean out of the window is dangerous.身子探出窗外很危险。
2.用it作形式主语，把真正的主语不定式置于句后，常用于下列句式中
（1）It+is+名词+to do
It is an offence to drop litter in the street.在马路上乱丢废物是违章的。
（2）It takes sb+some time+to do
How long did it take you to finish the work?你花了多少时间来完成这项工作？
（3）It+be+形容词+for sb+to do
It is stupid of you to write down everything (that) the teacher says.你把老师说的所有东西都记下来的行为是很愚蠢的。
（4）It seems(appears)+形容词+to do
It seemed impossible to save money.省钱似乎是不可能的。
（5）It + 不定式结构可位于believe/consider/discover/expect/find/think和wonder之后
He thought it would be safer to go by train.他认为乘火车比较安全。
（6）不定式的完成式也可作句子的主语
It is better to have loved and lost than never to have loved at all.爱过而后失去了爱，比从未爱过要好。
