动词不定式属于非谓语动词形式，在句中不能作谓语。通常动词不定式是由“不定式符号to 动词原形”构成的。动词不定式在句子中可以起以下作用：
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1.作主语。动词不定式作主语时，一般用it作形式主语，不定式后置，作真正的主语。To study English well is very necessary.= It is very necessary to study English well.学好英语是非常必要的。
2.作宾语。动词不定式作宾语时，其逻辑主语即为全句的主语。当复合宾语中的宾语是不定式时，常将形式宾语it放在谓语动词之后、宾语补足语之前，不定式放在宾补之后。The boy wants to ask his father a question.这个男孩想问他爸爸一个问题。I find it necessary to go there at once.我发现立刻去那里是必要的。
3.作宾语补足语。接带to的不定式作宾补的动词(词组)有ask,tell,want,wish,expect,would like等。接不带to的不定式作宾补的动词有hear，see，watch，notice，feel，let，make，have等。I asked him to go with me.我要求他和我一起去。I often hear her sing this song.我常听见她唱这首歌。
4.作表语。动词不定式作表语，位于连系动词之后。多数情况下，可转换为作主语。My duty is to keep the classroom clean.=To keep the classroom clean is my duty.我的职责是保持教室清洁
。5.作定语。不定式作定语时，位于被修饰的名词或代词之后。We have too much work to do this vacation.这个假期我们有太多工作要做。
6.作状语。不定式作状语往往表示目的、原因或结果，其逻辑主语通常是全句的主语。I am very glad to see you here.(表原因)我很高兴在这儿看见你。I went to the station to meet him.(表目的)我去车站接他。
拓展：①省略to的不定式结构had better do sth.最好做某事/let sb.do sth.让某人做某事/make sb.do sth.使某人做某事/Why not do sth.?=Why don't you do sth.?为什么(你)不做某事呢？②动词不定式的否定形式为not (to) do sth.。I always tell my students not to play on the road.我总是告诫学生不要在马路上玩耍。Could you please not put your bike here?请你不要把自行车放在这里好吗？
例题讲解
例1 (2014·北京中考)My parents always tell me more vegetables and fruit.A.eat B.eatingC.eats D.to eat解析tell sb.to do sth.为固定结构，意为“让某人做某事”，动词不定式用作宾语补足语。答案D例2 (2014·黑龙江绥化中考)We should do what we can our environment.A.protect B.to protectC.protecting解析结合选项可推知句意为“我们应尽我们所能去保护环境”。此处应用动词不定式表目的，故选B。答案B例3 We have two rooms ，but I can't decide .A.to live；to choose which one B.lived；choose which one C.to live in；which one to chooseD.live；which one解析句意：我们有两个房间可居住，但是我不能决定选哪个房间。第一个空是动词不定式作后置定语，修饰rooms；第二个空是动词不定式与疑问词连用构成不定式短语，作decide的宾语。答案C例4 The boy made us by making faces.A.to laugh B.laughingC.laugh D.laughed解析make后跟不带to的动词不定式作宾语补足语，故选C。
答案C
