在句中充當除謂語以外的句子成分的動詞形式叫做非謂語動詞。非謂語動詞分為三種形式：不定式、動名詞和分詞（分詞包括現在分詞和過去分詞）
一、 不定式的作用
1、 作主語不定式作主語時，謂語用單數。往往用it 作形式主語，把不定式放在謂語後面。如：
it took us two hours to finish the job.
it is impossible for us to get there on time.
it is very kind of you to help us.
注意：（1）其他系動詞如look, appear等也可用於此句型。
（2）當不定式作主語的句子中又有乙個不定式作表語時，不能用it is …to…的句型。試比較：
it is to negate my own idea to believe him.（錯）
to believe him is to negate my own idea .（對）
（3）it is+ adj. of / for sb. to do sth. 結構中，當不定式的邏輯主語和前面的形容詞可以構成係表結構時，用of, 否則用 for.
2、 作賓語
1. 動詞+不定式。如：
he managed to escape from the fire.
i find it hard to get along with him.（it 作形式賓語）
注：下列動詞通常用不定式作賓語：want, try, hope, wish, need, forget, know, promise, refuse, help, decide, begin, start, learn, agree, choose, get等
2. 動詞+疑問詞+to , 「特殊疑問句+不定式」相當於名詞，作賓語。如：
i don』t know what to do next/ how to do it next.
i can』t decide when to go there.
注意：不定式短語作賓語時，如果還帶有賓語補足語，往往把不定式短語放在賓語補足語之後，用it 作形式賓語。如：
i find it necessary to learn a foreign language.
3、 作賓語補足語
（1） 動詞+賓語+不定式（to do ）。如：
he warned me to be careful.
i want you to speak to tom.
what makes you think so?（不帶to的不定式）
注：可以用動詞不定式做賓補的動詞有：ask, tell, order ,want ,get, would like, like, advise, invite, allow, help, wish,warn, expect, prefer, encourage
（2） 表見解、看法的動詞結構可為：動詞+賓語+ to be 的不定式結構。如：
we consider tom to be one of the best students in our class.
the book is believed to be useful.（被動語態）
(3) there +不定式。如：
we didn』t expect there to be so many people there.我們沒料到會有那麼多人在那裡。
注意：（1）有些動詞需用 as 短語作補語，像regard, think, believe, take, consider。如：
we regard tom as our best teacher. 我們認為湯姆是我們最好的老師。
mary took him as her father. 瑪麗把他當作自己的父親。
（2）在動詞feel （一感），hear, listen to（二聽），h**e, let, make（三讓），notice, see, watch, observe, look at（五看）(即：吾看三室兩廳一感覺)等後面的補足語中，不定式不帶to，但變為被動語態後，必須帶to。如：
they saw the boy fall off the tree. the boy was seen to fall off the tree.
3)help後面作賓語補足語的不定式可以帶to,也可以不帶to.
i often help him （to）clean the room.
i helped him (to) find his things.
4、 作定語
不定式作定語，修飾名詞或代詞，不定式和它所修飾的名詞之間有邏輯上的主謂關係、動狀關係、同位關係或動賓關係。不定式通常要放在這些被修飾的詞後。如：
i h**e a lot of work to do.（動賓關係）
he is looking for a room to live in.（動狀關係）
he is the first person to think of the idea.（主謂關係）
he has got a chance to go abroad.（同位關係）
注意：1.不定式的邏輯主語是句子的主語時，不定式用主動形式表被動。如：
do you h**e anything else to say?
2.如果作定語的不定式是乙個短語，則要保留不定時短語中的副詞或介詞。如：
i need a pen to write with . (i will wirte with the pen ) (我需要一直鋼筆寫字)
i h**e a little baby to look after .(i must look after the little baby ) (我有乙個嬰兒要照看)
作狀語，表示目的、結果、原因等，有時還有一些固定搭配的不定式短語，如in order to , so as to, so…as to, such …as to, ….enough to, too…to等。
（1） 做目的狀語，to, only to （僅僅為了）, in order to, so as to, so(such)….as to…（如此···以便···）如：
he ran so fast as to catch the first bus. 他飛快地跑以便趕上第一班車。
he came to the school to see his son.
（2） 作結果狀語，表事先沒有預料到的，要放在句子後面。如：
he hurried to the post office only to find it was closed.
he searched the room only to find nothing.
（3） 做原因狀語。如：
we were very excited to hear the news.
i』m glad to see you.
（4） 做條件狀語。如：
to turn to the left , you could find a post office.
5、 作表語
不定式可放在be動詞後面，構成表語。如：
the question is how to put it into practice.
my question is when to le**e.
his dream is to be a doctor.
her work is to look after the babies.
注意：1.不定式在句中作表語時，對應的謂語動詞用單數。
2.當助於是不定式時，表語不能用ving形式，可用不定式。
如：to see is to believe. （眼見為實）
6、 獨立結構。如：
to tell you the truth, i don』t agree with you.
to make matters worse, it began to rain.
二、 不定式的時態和語態
1、 不定式的時態
（1） 現在時：有時與謂語動詞表示的動作同時發生，有時發生在謂語動詞的動作之後。如：
he seems to know this. i hope to see you again.
（2） 完成時：表示的動作發生在謂語動詞表示的動作之前。如：
i』m sorry to h**e given you so much trouble.
he seems to h**e caught a cold.
（3） 進行時：表示動作正在進行，與謂語動詞表示的動作同時發生。如：
he seems to be eating something.
（4） 完成進行時：表示動作從過去某個時間開始，一直延續到現在，並有可能持續下去。如：
she is known to h**e been working on the problem for many years.
2、 不定式的語態
當不定式的邏輯主語是其動作的承受者時，就用被動式。如：
he was seen to enter the hall. he asked to be sent to work in tibet.
三、 省to 的動詞不定式
1、 情態動詞（除ought 外，ought to）
2、 would rather, had better.
3、 感官動詞 see, watch, look at, notice, observe, hear, listen to, smell, feel 等後作賓補，省略 to.
注意：在被動語態中 to 不能省略掉。如：
i saw him dance. he was seen to dance. the boss made them work the whole night.
they were made to work the whole night.
4、 使役動詞 let, h**e, make.
5、 由 and, or 和 than 連線的兩個不定式，第二個 to 可以省去。如：he wants to move to france and marry the girl.
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