在语法中，动词不定式是指动词的一种不带词形变化从而不指示人称、数量、时态的形式。它之所以被叫做不定式，是因为动词不被限定，或者说不被词形变化所局限。
不定式属于非谓语动词。然而在一些语言里（例如葡萄牙语），存在着受时态、人称和数量影响的不定式形态。
用法：
动词不定式作宾语
不定式可以充当部分及物动词的宾语，也可以充当but和except等介词的宾语以及形容词的宾语.
1. 作动词的宾语
①不定式可以充当部分及物动词的宾语
Father likes to listen to music in silence. 父亲喜欢静静地听音乐。
He prefers to be starved to death rather than beg. 他宁愿饿死也不原乞讨。
I rather thought to meet you here. 我没想到在这里遇见你。
②在feel, find, make, think, consider, 等动词后，如果宾语带有宾语补足语时，人们通常用it作形式宾语，而把真实的宾语放在宾语补足语之后。
I found it possible to work out the problem without a computer. 我发现有可能不用计算机而解出这道题目。
She made it a rule to get up at 5:00. 她养成了五点起床的习惯。
I feel it a great honor to be invited to speak at the meeting before so many students.我觉得被邀请在会上面对这么多学生发言是一件很光荣的事情。
③在表示“希望、打算”等动词（如hope, except, intend, mean, want, 等）的过去式后，可接动词不定式的完成式来表示没有实现的动作。
I intended to have called on you ,but was prevented from dong so. 我原想来拜访你的，但没来成。
I expected to have met him here last night. 我以为昨天晚上能在这里见到他。（但没做到）
We meant to have stayed there a week. 我们原打算在那儿呆一个星期的 。
I wanted to have dropped her a line, but forgot to do so. 我原想写一封信给她，但忘记了。
2. 作介词的宾语
不定式可以作介词but和except的宾语。
The teacher made no comments except to tell him to work hard. 老师除了叫他努力学习外，未做任何评价。
He had no choice but to sit there as usual. 他没有什么选择，只好像往常一样坐在那儿。
There is nothing we can do but wait patiently. 我们只能耐心等待。
3. 作形容词的宾语
不定式作形容词的宾语有两种类型，一是句子的主语是不定式的逻辑主语，二是句子的主语是不定式的逻辑宾语。
①句子的主语是不定式的逻辑主语
这一类的形容词有：afraid, angry, anxious, clever, content, cruel, determined, disappointed, eager, foolish, fortunate, frightened, happy, impatient, glad, lucky, naughty, prepared, proud, ready, slow, shocked, sorry, surprised, willing等。
I am sorry to say that he is going from bad to worse. 很遗憾他的情况每况愈下。
She was not content to live a quiet life in a small town. 他不满足在一个小镇里过默默无闻的生活。
②句子的主语是不定式的逻辑宾语
这类的形容词有：easy, hard, cheap, expensive, dangerous, difficult, funny, fit, impossible, interesting, nice, pleasant, simple, strange, useful,等.
This problem is easy to solve. 这个问题很容易解决。
The water is not fit to drink. 这水不适合饮用。
She is hard to get along with. 她不太好相处。