不定式在句中做定語通常做后置定語，即放在被修飾名詞或代詞后。它通常有以下幾種用法：
(一)不定式常可用來修飾物，表示要做某事。
如：1、The next train to arrive was from NewYork.下一列到站的火車是從紐約開來的。
2、I have nothing to say on this question.在這個問題上，我沒有什么話要說。
3、We are looking for somewhere to live.我們正在找一個地方住。
(二)不定式還可用來修飾人。
如：1、He was the first guest to arrive.他是第一個到達的客人。
2、Miss Brown was the next person to rise to speak.布朗小姐是下一個起來發言的人。
3、He was a brave man to do what he did.他是個勇者才有這樣的行為。
(三)很多特定的名詞后也可用不定式做定語，這樣的名詞常用的有：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.你該起床去上學了。
2、There is no reason to doubt his word.沒有理由懷疑他的話。
3、They have now an opportunity to go abroad to study further.他們現在有機會出國深造。
(四)某些動詞或形容詞后可接不定式時，它相應的同根名詞也常用不定式做定語。
如：1、He made an attempt to learn English well.他試圖學好英語。(attempt to do sth)
2、His ability to get on with people is his chief advantage.他能和人相處是他的主要優勢。(be able to do sth)
3、I don't trust his promise to come for a visit.我不相信他來訪的諾言。(promise to do sth)
4、Her anxiety to succeed led her to work hard.她迫切想成功，這促使她努力工作。(be anxious to do sth)