动词不定式（to do）在句中可作主语、宾语、定语、状语等。口诀：to 加原形不定式，可以充当名形副。
顺口溜记忆
- to 加原形不定式，词组可做名、形、副
- want、hope 和 wish，agree、decide 后面跟
- 作宾语常接 to do，固定搭配要记清
- 作目的状语放句首，为了某事 To do 开头
- 感官使役动词后，不带 to 要记住
要点讲解
- 不定式作宾语：want to do, hope to do, decide to do, agree to do, plan to do, learn to do
- 不定式作目的状语：To catch the bus, he got up early. 为了赶上公交车，他起得很早。
- 不定式作定语：I have something to say. 我有话要说。
- 不定式作主语：To learn English well is important. = It is important to learn English well.
- 感官动词（see/watch/hear/feel）和使役动词（make/let/have）后接不定式作宾补不带 to：I saw him leave.
例句解析
- She wants to be a doctor. 她想成为一名医生。
- He decided to study abroad. 他决定出国留学。
- I went to the library to borrow some books. 我去图书馆借了一些书。
- The teacher told us to keep quiet. 老师叫我们保持安静。
易错提醒
- ask/tell/want 等动词后接"宾语 + to do"：ask sb. to do sth.；其否定形式是 ask sb. not to do sth.。