不定式在句中做定语通常做后置定语，即放在被修饰名词或代词后。它通常有以下几种用法：
(一)不定式常可用来修饰物，表示要做某事。
如：1、The next train to arrive was from NewYork.下一列到站的火车是从纽约开来的`。
2、I have nothing to say on this question.在这个问题上，我没有什么话要说。
3、We are looking for somewhere to live.我们正在找一个地方住。
(二)不定式还可用来修饰人。
如：1、He was the first guest to arrive.他是第一个到达的客人。
2、Miss Brown was the next person to rise to speak.布朗小姐是下一个起来发言的人。
3、He was a brave man to do what he did.他是个勇者才有这样的行为。
(三)很多特定的名词后也可用不定式做定语，这样的名词常用的有：way，time，reason，chance，opportunity，need，wish，effort，right，ambition等。
如：1、It's time for you to get up and go to school.你该起床去上学了。
2、There is no reason to doubt his word.没有理由怀疑他的话。
3、They have now an opportunity to go abroad to study further.他们现在有机会出国深造。
(四)某些动词或形容词后可接不定式时，它相应的同根名词也常用不定式做定语。
如：1、He made an attempt to learn English well.他试图学好英语。(attempt to do sth)
2、His ability to get on with people is his chief advantage.他能和人相处是他的主要优势。(be able to do sth)
3、I don't trust his promise to come for a visit.我不相信他来访的诺言。(promise to do sth)
4、Her anxiety to succeed led her to work hard.她迫切想成功，这促使她努力工作。(be anxious to do sth)