1. 作主语，如：
　　To master a foreign language is no easy job.

　　但在当代英语中，不定式作主语置于句首的不多，常用先行词it 作形式主语，而将真
　　实主语不定式移置谓语之后，即用句型"It +(be)+ adj./n.+ to v"，如：
　　It is foolish to make a decision without knowing all the facts.
　　It's a pleasure to talk with you.

　　如果是疑问句或感叹句，则只能用上述句型，如：
　　How much did it cost you to repair the car?
　　What a magnificent view it was to overlook the valley at the top of the mountain!

2. 作表语，如：
　　The most important thing is to put theory into practice.

　　不定式作表语通常要带to，但当主语部分含有do的某种形式时，to可省略，如：
　　All we have to do is push the button.