非谓语动词（一）：不定式

本节导读

非谓语动词是动词的另一类形式，不能单独作句子的谓语，却能在句子里充当主语、宾语、定语、状语等成分。它包括不定式、动名词和分词三种，是词汇与语法部分的重点，也是不少同学觉得最乱的地方。

这一节先认识三种非谓语动词，再重点讲不定式；下一节讲动名词和分词。学完这一节，你应该能判断一个动词后面该接不定式还是动名词，并掌握不定式在句中的各种用法。

三种非谓语动词

形式	构成	例子	常见作用
不定式	to + 动词原形	to read	主语、宾语、补语、定语、状语
动名词	动词 + -ing	reading	主语、宾语、表语
现在分词	动词 + -ing	reading	定语、状语、补语
过去分词	动词 + -ed 等	read	定语、状语、补语

不定式还有被动形式（to be done）和完成形式（to have done）：He hopes to be invited. She is said to have lived in Paris.

不定式作主语和宾语

作主语时常用 it 作形式主语，把真正的主语放到后面：To learn English well is not easy. 通常写成 It is not easy to learn English well.

句型	例句
It is + 形容词 + for sb. to do	It is important for us to keep fit.
It is + 表示人品质的形容词 + of sb. to do	It is kind of you to help me.

有些动词后面只能接不定式作宾语，常见的有：want、hope、wish、decide、plan、expect、refuse、manage、afford、learn、pretend、promise、offer、agree、fail、choose。

英语	中文
He decided to study abroad.	他决定出国留学。
She refused to answer the question.	她拒绝回答这个问题。
We cannot afford to buy a car.	我们买不起车。

不定式作宾语补足语

ask、tell、want、allow、advise、invite、order、expect、encourage、force 后面用「宾语 + to do」：The teacher asked us to read the text.

下面几类动词后面的宾语补足语不带 to：

动词	结构	例句
make, let, have	+ 宾语 + 动词原形	The teacher made us read the text.
see, hear, watch, notice, feel	+ 宾语 + 动词原形（强调全过程）	I saw him cross the road.
help	+ 宾语 + (to) do	She helped me (to) carry the bag.

这些词变成被动语态时，原来省略的 to 要补出来：He was seen to cross the road. They were made to work.

不定式作定语和状语

作定语时放在被修饰的名词后面：I have something to tell you. 在 the first、the last、the only 或序数词修饰的名词后，也常用不定式：He was the first student to arrive.

作状语时有几种含义：

含义	例句
目的（可加 in order to 或 so as to）	He studies hard to pass the exam.
结果（only to do 表示出乎意料的结果）	He hurried to the station, only to find the train had left.
程度（too…to, enough to）	He is too tired to walk. This room is big enough to hold fifty people.
原因	I am glad to see you again.