在英语中，单独的某个动词v.只能用作谓语，要想让v.能充当主语、宾语、表语、补语、定 语、状语、同位语和独立成分等角色，就得改变v.本来的形式，方法就是采用动词不定式（to v)、动名词(v-ing)、现在分词(v-ing)或过去分词（v-ed)的形式。此时的v.意思未变，但词性 发生了变化，故可以充当其他成分。
不定式的基本形式是“to +动词原形”，有时to可以省略，多数情况下有表示“将来”之 意。不定式具有名词（主、宾、表和同位语），形容词（定语和补语）和副词（状语和独立成分）的 基本特征。因此，它可以在句子中充当的成分十分灵活。此外，特殊疑问词+不定式=复 合不定式，复合不定式仅具有名词的基本特征。
To ignore this would be a mistake.
It made us very angry to hear him talk like that.
It is more important not to think lies.
l‘m longing to see Judy again.
They found it impossible to get everything ready in time.
We thought it unnecessary to argue with him about it.
My mother will teach me how to cook the dish.(复合不定式）
He didn’t know whether to laugh or to cry when he heard the words.(复合不定式）
Our task is to clean the windows.
That boy seems to be a college student.
All you can do now is wait until we come.(注:不带to的不定式作表语，原因是前面有do， 前后要相互对应）
His ambition，to be a pilot，was never fulfilled.
We'll soon have to face that annual problem—what to give her for her birthday.
I have something to do.
She has a lot of things to tell you.
Let’s first find a room to put the things in.
Linda asked Tom to buy her a book written by the essayist.
I often hear him sing this song.(不带to的不定式做宾补，因为hear是感官动词）
Don’t forget to have him come.(不带to的不定式做宾补，因为have是使役动词）
Someone was heard to come up the stairs, (hear是感官动词，但用于被动时前面要加to，此处作主补）
They were made to work day and night, (make是使役动词，但用于被动时前面要加to，此处作主补）
She came to see me yesterday.(目的）
We were surprised to hear the news.(原因）
He is old enough to join the army, (结果)
To tell you the truth，I am not in favor of your proposal.
He was rude，to say the 丨east of it.