不定式作定语、状语、补语
不定式还能修饰名词，或表示目的、结果，也能补充说明宾语。
something to drink / too … to … / ask sb. to do
要点
- 作定语放在名词后面：something to drink、a lot of work to do。
- 不定式若是不及物动词，后面要补上介词：a room to live in。
- 作状语常表目的（to / in order to / so as to）或结果（too … to、enough to）。
- 作宾补：ask / tell / want sb. (not) to do。
- 被 the first / the only / 最高级 顶住的名词，后面习惯接不定式而不接 -ing：the first person to reach the top、the best way to learn English.
- 表"结果"的不定式常跟在 only 后面，而且那个结果多是意外或落空：He hurried home only to find the door locked.（赶回家却发现门锁着）
- 宾补位置上是"形容词 + to do"时，要拿 it 顶住：We thought it necessary to tell him at once. —— 不说 thought necessary to tell him。
- 不定式也有"更早一步"的形式 to have done：He is said to have retired years ago.（主句说现在，动作在过去就得补 have done）。
例句
I would like something to drink.
我想要点喝的。
She was brave enough to face the danger.
她足够勇敢面对危险。
He is too shy to speak aloud.
他太害羞不敢大声说话。
The driver asked us to stay in our seats until the bus stops.
司机让我们坐在位子上，等车停了再起来。
He hurried home only to find the door locked.
他赶回家，却发现门锁着。
We thought it necessary to tell him at once.
我们认为有必要马上告诉他。
易错提示
- 不定式作定语别丢介词：a pen to write with（不是 a pen to write）。
- too … to 本身已含否定，后面不要再加 not。
- 表目的可说 to / in order to / so as to，但 so as to 不能放在句首：句首表目的写 In order to catch the train, he got up early.；用 So as to 起头就不行（so as to 只待在主句后面）。