动词不定式用作主语。此时，谓语动词用第三人称单数形式。需要注意的是，如果主语较长，可以用it做形式主语，而将真正的主语动词不定式放在句尾。
02动词不定式用作表语。例如：The most important thing is to believe her.
03动词不定式用作宾语。例如：I like to eat apples.
04动词不定式用作宾语补足语。例如：He tells me to write it like this.
05动词不定式用作定语。例如：I have lots of things to do.
06动词不定式用作状语。例如：It is too late to leave there.
07特殊疑问词＋动词不定式。例如：I don't know how to do it.
08动词不定式有时不带to.例如：He saw a cat enter the box.
