动词不定式是一种非谓语动词形式，其结构为“to＋动词原形”，其中to是其符号，它没有人称和数的变化。动词不定式在句子中可以用作主语、表语、宾语、补足语、定语、状语等。
· To be a doctor is hard. 做医生很难。
· To learn English well is not easy. 学好英语不容易。 · To say is to believe.眼见为实。动词不定式（短语）作主语时，为了避免句子的头重脚轻，可以用“it”作形式主语，而把真正的主语－－动词不定式（短语）放在句子的后部。· It is hard to be a doctor. · It is not easy to learn English well. · It’s important to plant trees in spring. 如果动词不定式的动作是谁做的，可以在动词不定式前面加一个由介词for引导的短语，称为不定式的复合结构。· It’s good for us to read English aloud in the morning. 对我们来说每天早晨大声读英语是很有益的。· It is important for students to use English every day. 对学生来说每天使用英语是很重要的。在形式上，位于系动词后的就是表语。从含义上讲，表语是回答主语“是什么”或“怎么样”的语法成分。动词不定式（短语）作表语常用于系词be的后面。· His work is to drive a car. 他的工作是开车。· My job is to feed animals. 我的工作是饲养动物。· Her ambition is to be a doctor. 她的理想是当医生。动词不定式（短语）可以放在一些动词后面用作宾语，能以动词不定式作宾语的动词有：begin, start, want, forget, remember, show, learn, like, hate, love, ask等。· I want to tell you a story. 我想给你讲个故事。 · They begin to work at eight every morning. 他们每天早晨8点开始工作。 · Don’t forget to lock the door.别忘了锁门。 · Would you like to go and have a picnic with us tomorrow?明天和我们起去野餐好吗？1.如果and连接两个动词不定式，第二个动词不定式一般省“to”。· He wants to go and have a swim with us. 他想和我们一起去游泳。2.若作宾语的动词不定式（短语）很长，可用it作形式宾语。· I find it interesting to learn English with you. 我觉得和你一起学英语很有趣。 · He found it hard to catch up with others.他觉得赶上别人很困难。· The teacher asked us to read English for half an hour in the morning.老师让我们早晨读半小时英语。 · The teacher often tells Jim not to spend too much time playing computer games.老师常告诉Jim不要花太多时间玩电脑游戏。 · Please let me help you.让我来帮助你。 动词不定式作宾语补足语，表示宾语所做的动作，可以用动词不定式作宾语补足语的动词有：tell, ask, want, like, invite, encourage, help等。但在let, make, see, watch, hear, feel, notice, have等感官听觉动词或使役动词后面作宾语补足语的动词不定式一般要省略动词不定式符号“to”，可以归纳为以下三种句式：不可以省to的不定式作宾语补足语：tell /ask / want / would like / wish / like / invite /encourage /teach sb. to do sth. The teacher told us to come earlier tomorrow. I want you to go now. 我想让你现在就走。 Her parents wish her to be a teacher.她父母亲希望她当老师。 省“to”的不定式作宾语补足语：Let / make / have sb. do sth. Let the boy go out now. 让那个男孩出去。 The boy made the baby cry. 男孩弄哭了这个小孩。 see / watch / hear / notice / feel sb. do sth. I saw the students play basketball on the playground yesterday.昨天我看见学生们在操场打篮球。 I often hear the girl sing in the next room. 我经常听到女孩在隔壁唱歌。可省可不省的：help sb. （to）do sth. I often help my mother （to）do housework. 我经常帮妈妈做家务。 （1）动词不定式的否定式是直接在to之前加not。· Tell them not to play football in the street.告诉他们不要在街道上踢足球。 （2）省to的动词不定式的否定式是直接在动词前面加not。· Let the boy not go. 让那个男孩别走。 （3）如果将主动语态变为被动语态时，作宾语补足语的动词不定式变为主语补足语，动词不定式则不省“to”，即原来省的再加上。· The boy made the baby cry. · The baby was made to cry by the boy.那个孩子被男孩弄哭了。动词不定式作定语常用来修饰名词或不定代词，放于所修饰的词后，为后置定语。 · Who was the first one to arrive? 谁第一个到的？ · She has no paper to write on? 她没有纸写字？ · The best way to learn English is to use it. 学英语最好的方法是使用它。 · When is the best time to plant vegetables?什么时候是种植蔬菜的最好时间？· Do you have something to drink? 你这有喝的吗？动词不定式（短语）作定语修饰名词时，与其所修饰的词具有一种动宾关系，如果不定式中的动词为不及物动词时，其后应加上一个含义上所需要的介词。· I have no chair to sit on.我没有椅子坐。 · He has no house to live in.他没有房子住。动词不定式作目的状语常用在go, come, hurry等不及物动词后，表目的。· He went to see his sister yesterday.他昨天去看妹妹了。 · They got up early to catch the early bus.他们早起去赶早班车。有时可以把不定式（短语）提前，放于句首，表示强调目的。· To learn English quickly and well, he went to England.为了更快更好地学英语，他去了英国。
