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英语什么情况下加动词不定式
一、不定式等非谓语形式作主语时，谓语动词一路走来都是单数身份；不少作主语的不定式与动名词在意义上相同。如：
To see is to believe. = Seeing is believing. 眼见为实。
二、口语中，当作主语的不定式过长时，it不惜屈居有名无实的形式主语，常为平衡关系而挺身而出、替主出征，"真主角"反无官一身轻，悠哉断后。如：
To learn a foreign language well is not easy.→ It's not easy to learn a foreign language well. 要把一门外语学好不容易。
To play football is very interesting. → It's very interesting to play football. 踢足球很有趣。
留言提醒：在实际操作中，不定式（短语）还有权"招聘"一个逻辑主语，并由of / for出面"料理"--It's + adj. + for / of sb. + to do sth.，下例中的"for me"即是：
It's hard for me to finish the task in such a short time. 对我而言，要在如此短的时间里完成任务是困难的。
聘of还是for？点击作表语的形容词属性可知：若形容词描述的是人的特征与品行，能修饰人（像kind，
nice， good， clever， right， polite，
careless等），则"of适宜"；如果形容的是事物的特点或外观等（如easy，hard / difficult， safe，
interesting， important，necessary之类），则"for主动上"！下例"正误"可略见一斑：
出席这个会议对你来说很重要。
[误] It's very important of you to attend the meeting.
[正] It's very important for you to attend the meeting.
留言提醒：检验形容词描绘的是不是人的特征与品行、能否修饰人--"sb. be adj."可助上一臂之力。
三、一个都不能少：还有几种耳熟能详的"不定式作主语"句式。如：
1. It takes / took / will take (sb.) some time to do sth.如：
It takes (us) hours to do our homework every day. 我们每天要花上数小时做家庭作业。
2. It's + n. + to do sth.如：
It's a mistake for us not to help him. 我们不帮他是个错误。
留言提醒：要对不定式否定，暗号照旧--not在不定式前面加！
3. It cost / costs sb. some time / money to do sth. 如：
It cost me ten yuan to buy this bag. 我花了十元钱买这个书包。
四、时代流行风--主语也能由疑问词how / what / when / where / which等携手与不定式"联袂出演"。如：
Where to put the bikes is being discussed. 把自行车放哪儿正在讨论当中。
When and where to have the meeting hasn't been known. 什么时候、什么地方开会还是未知数。
什么时候用动词不定式作表语
不定式可以作表语，动名词也是可以作表语的，二者作表语的时候既有相同点也有不同的地方。
一、不定式和动名词作表语相当于一个名词作表语，含义是回答主语“是什么”。如：
1. Our plan is to keep the affair secret.
我们的计划是让这件事成为秘密。（主语和表语是相等的）
2. Their job is making wheelchairs for disabled people.
他们的工作是为残疾人制造轮椅。（主语和表语是相等的）
二、不定式和动名词作表语虽都是用于回答主语“是什么”的，但二者仍有一些区别。不定式作表语强调的是一次性、具体的、将要发生的动作；动名词作表语强调的是一般性、抽象的、经常发生的动作。如：
1. His job is to paint the walls.
他的工作是粉刷这些墙。（一次性的，具体的，目前要做的事情，并不是他日常的工作）
2. His job is painting walls.
他的工作是粉刷墙。（这是他日常的工作）
不定式的逻辑主语
不定式英语不定式的逻辑主语有下面几种情况。
一. 不定式作主语时有多种情况。最明显的是用for引出不定式的逻辑主语.
a) It’s necessary for us to learn English well.
b) It’s impossible for them to finish learning the two books in only one month.
c) It’s a mistake for learners to ignore grammatical rules when learning a foreign language.
二. 不定式作宾语时,其逻辑主语是它句子本身的主语。
a) You won't want to be used at this time.(不定式的逻辑主语:you)
b) He wants to go with us. (不定式的逻辑主语:he)
c) She hopes to have an English dictionary. (不定式的逻辑主语:she)
三. 不定式作表语时，其逻辑主语通过主语部分的某一个名词或代词体现出来。
a) Her aim is to make the body stronger.(不定式的逻辑主语通过her体现出来：she)
b) His work seemed to communicate news of food. (不定式的逻辑主语通过his体现出来：he)
c) No one appeared to notice me. (不定式的逻辑主语就是no one)
四. 不定式作定语时。
除非特别用for……表示不定式的逻辑主语，否则，不定式做定语时，不定式的逻辑主语多是被不定式修饰的那个名词。
1. 不定式在主语部分中作定语。
1) The problem to be discussed is on the piece of paper.（不定式的逻辑主语是the problem）
2) The writer to come tomorrow is from beijing. （不定式的逻辑主语是the writer）
3) There are still some trees to be planted. （不定式的逻辑主语是some trees）
4) The trees for you to plant will be here tomorrow. （不定式的逻辑主语是you）
2. 不定式在表语部分中作定语。
a) She is not always the first to come to school. （不定式的逻辑主语是the first）
b) They are the boys to go to play football in the USA. （不定式的逻辑主语是the boys）
c) This is the problem to be discussed. （不定式的逻辑主语是the problem）
d) They are not the exercises to be done this evening. （不定式的逻辑主语是the exercises）
e) She was the first person to think of the idea. （不定式的逻辑主语是the first person）
f) China was one of the first countries in the world to study the science of farming. （不定式的逻辑主语是the first countries in the world）
g) My father says that engineering is not a suitable subject for a girl to study. (不定式的逻辑主语是a girl)
h) This is the book for you to read. （不定式的逻辑主语是you）
3. 不定式在状语部分中作定语。
1) Are they still living in the room to be repainted? （不定式的逻辑主语是the room）
2) He is living in the same room with a prisoner to be sentenced to death. （不定式的逻辑主语是a prisoner）
4. 不定式在宾语部分中作定语,有以下两种情况。
1) 如果句子本身的主语是不定式所表达的动作的执行者，它同时就也是不定式的逻辑主语。
a) The student wants a book to read this afternoon.（不定式的逻辑主语是the student）
b) Does Mary have a classmate to go (to Shanghai)with? （不定式的逻辑主语是是Mary）
c) We have no songs to sing after class.（不定式的逻辑主语是We）
d) They have nothing to eat. （不定式的逻辑主语是They）
e) He had some letters to write. （不定式的逻辑主语是He）
f) He bought a book for his daughter to read. （不定式的逻辑主语是his daughter）
2) 如果句子本身的主语不是不定式所表达的动作的执行者，不定式的逻辑主语是被不定式修饰的名词。
He made some candles to give light. （不定式的逻辑主语是some
candles）
3) 在含有双宾语的句子中，情况比较特别。不定式所表达的动作的执行者,就是指代人的那个宾语，是不定式的逻辑主语。
a) Please give him the watch to repair. （不定式的逻辑主语是him）
b) Please lend me a book to read. （不定式的逻辑主语是me）
c) This work offered him some money to live on. （不定式的逻辑主语是him）
五. 不定式作状语时，其逻辑主语都是句子本身的主语。
a) I stayed there to see what would happen.(目的状语to see what would happen的逻辑主语是“I”)
b) He will go to the clinic tomorrow to be examined by the doctor.( 目的状语to be exemined的逻辑主语是“He”)
c) He lived to be a very old man.( 结果状语to be a very old man的逻辑主语是“He”)
d) In 1935 he left home never to return.( 结果状语to return的逻辑主语是“he”)
e) I trembled to think of it.( 原因状语to think of it的逻辑主语是“I”)
f) She wept to see him in such a terrible state.( 原因状语to see him in such a terrible state的逻辑主语是“She”)
六. 不定式作补语时，其逻辑主语是主动句中的宾语或被动句中的主语.
1) None of them requires the equipment to be moved. （不定式的逻辑主语是the equipment）
2) Women were not allowed to take part in the games.(不定式的逻辑主语:women)
3) She is the engineer whose work is considered to be the best here.(不定式的逻辑主语: whose work)
4) I'll ask them to be quiet.(不定式的逻辑主语: them)
七. 不定式作同位语时，其逻辑主语只能在语言环境中体现。
1) He gave the order to start the attack.(本句省略了“for somebody”, to start the attack 的逻辑主语是somebody)
2) Her suggestion to plant some trees and flowers was accepted.（to plant some trees and flowers的逻辑主语视场合决定）
不定式做主语和动名词做主语的区别
用staying
，feel
like后面只能接ing形式，不接不定式的
不定式和动名词做主语的区别:
不定式和动名词都可在句子中作主语，一般说来,
有下面几点须给予注意：
(1)
不定式一般表示具体的、特定的行为；而-ing分词一般表示抽象的、一般的行为。如：
To
complete
the
program
needs
much
effort.
完成这项计划需要很大的努力。
Riding
a
bike
can
make
a
man
smart.
骑单车能使一个人敏捷。
(2)
在疑问句中，只能用动名词的复合结构作主语。如：
Is
his
speaking
reasonable?
她的说话有道理吗？
(3)
在某些句型中通常要用动名词短语作主语。
如：It
is
no
use
doing…/
It
is
no
good
doing…
/
It
is
useless
doing…/
It
is
worthwhile
doing…
如：It
is
no
use
talking
your
trouble
to
a
man
like
him.
跟他那样的人谈你的烦恼没有用。
(4)
一般说来，作主语和表语的非谓语动词形式应一致，即：或同时为不定式，或同时为-ing分词。如：
Seeing
is
believing./To
see
is
to
believe.
眼见为实
当表语用来说明主语的特定意义或具体内容时，主语和表语的非谓语动词的形式可以不一致，即，主语用动名词，而表语用动词不定式。如：
Waving
your
hand
is
to
say
“Good-bye”.
挥手就是说“再见”。
Nodding
your
head
is
to
say
“Yes”.
点头就是说“对，是的”。
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