1、不定式的句法作用
(1)作主语。动词不定式作主语时,句子的谓语动词常用单数,其位置有以下两种:
<1>把不定式置于句首。例如:
To get there by bike will take us half an hour.
<2>用 it 作形式主语,把真正的主语不定式置于句后。例如:
It’s our duty to take good care of the old.
(2)作宾语。
<1>动词 + 不定式。例如:
The driver failed to see the other car in time.
<2>动词 + 疑问词 + to do。例如:
Please show us how to do that.
当复合宾语中的宾语是不定式时,先用形式宾语 it 代替不定式,再把不定式置于补语之后,即主语 + 动词 + it + 补语 + to do 句式。例如:
We think it quite important for us to learn a foreign language well.
(3)作表语。不定式作表语表示具体动作或将来动作。例如:
My chief purpose is point out the difficulties of the matter.
(4)作定语,不定式在句中作定语,置于被修饰的名词或代词之后。例如:
The next train to arrive is from Washington.
(5)作状语,表示目的、结果或条件,如表目的:to…only to(仅仅为了),in order to,so as to,so(such)…as to…(如此…..以便…..)。例如:
He ran so fast as to catch the first bus.
(6)作补语。
<1>动词 + 宾语 + 不定式。例如:
Father will not allow us to play on the street.
<2> to + be 的不定式结构。例如:
We consider Tom to be one of the best students in our class.

2、不定式的进行时和完成时
进行时:表示动作正在进行,与谓语动词表示的动作同时发生。例如:
He seems to be eating something.
完成时:表示的动作发生在谓语动词表示的动作之前。例如:
I’m sorry to have given you so much trouble.
He seems to have caught a cold.

3、不定式的被动语态
如果动词不定式的逻辑主语是这个不定式所表示的动作的承受者,不定式要用被动语态形式。例如:
It’s a great honor to be invited to Mary’s birthday party.

4、不定式的否定形式
其否定形式为:not to do。例如:
She pretended not to see me when I passed by.

5、不带 to 的动词不定式
(1)情态动词(除 ought to 外)
(2)使役动词 let,have,make。
(3)在感官动词 see,watch,look at,notice,observe,hear,listen to,smell,feel,find 等后作宾补时,不定式省略 to。注意:在被动语态中 to 不能省掉。
(4)would rather,had better 句型。
(5)“why…/ why not…”句型。
(6) help 可带 to,也可不带 to,如 help sb (to) do sth。
(7) but 和 except:but前是动词 do 时,后面出现的动词用不带 to 的动词不定式。
(8)由 and,or 和 than 连接的两个不定式,第二个 to 可以 省去。
(9)通常在 discover,imagine,suppose,think,understand 等词后,可以省去 to be。例如:
He is supposed (to be) nice.