不定式在句子中可做主語、賓語、表語、定語、狀語和賓語補語。
1)不定式做主語一般表示具體的某次動作。例如：
to complete the 30瞫toried building in one year was quite a difficult task.
to do that implies taking responsibility.
當主語較長，謂語較短時，常用it做形式主語，而將不定式放到謂語的後面。例如：
it is important for modern young people to master at least two foreign languages.
it做形式主語，不定式放在謂語動詞之後常用於下列結構中：
a) it is＋形容詞(easy, important, difficult, foolish, inconvenient, unnecessary, right, wrong...)＋( for/of sb.)＋不定式
it is essential to reserve a table in advance of christmas eve.
it is hard to put my hopessintoswords.
it is not easy to catch fish with your hands only.
it is important for us young people to learn english and master it.
b) it is＋名詞(a pleasure, a pity, a pleasant thing, one’s du瞭y, an honor, a shame, a crime, no easy job...)＋不定式
it is a sheer waste of time to read that kind of trash.
it is a pity to have to go without her.
it is a glorious death to die for the people.
c) it takes (sb.) some time (hours, months, days, a lot of time, patience...)＋不定式
it takes me three hours to learn english each day.
it took them half the night to get home in the snow.
2)不定式作表語不定式作表語常表示將來的動作，主語常常是表示意向、打算、計畫的詞，如wish, idea, task, purpose, duty, job等。
the most important thing for one’s health is to have plenty of exercise.
my chief purpose has been to point out the difficulties of the matter.
the purpose of the organization is to greet all new comers to the city and to provide them with any necessary information.
what i wanted was to get the work done as quickly as possible.
3)不定式作賓語不定式常在下列動詞後面作賓語： afford, agree, apply, arrange, ask, attempt, beg, begin, care, choose, claim, consent, demand, decide, desire, determine, expect, fail, hope, hesitate, hate, intend, learn, like, manage, mean, neglect, offer, plan, prepare, pretend, promise, refuse, resolve, seek, tend, threaten, want等。
i like to go out for walks in the warm sunshine in spring.
my mother hates to move from place to place, for she feels tired.i decide to work hard and get doctor’s degree.
the room is designed to be my study, but now it has to be used as a bedroom for the children.
she failed to finish the assignment in time, and she was worried about it.
4)常見的可以接這種不定式短語的動詞有：know, see, decide, tell,ask, consider, discover, explain, findout, forget, guess, hear, imagine, inquire, learn, observe, perceive, remember, think, understand, wonder等。
i couldn’t decide which book to choose.
i can tell youswheresto get this book.
they found it hard to decide whether to go swimming next sunday or to visit aunt sally in new year.
“wh-word+不定式”結構除了作賓語外，還可以在句中作主語或者表語。
when to start the program remains undecided. the question is how to put the plansintospractice.
5)不定式作狀語
a)不定式作狀語表示目的。或用於so as和in order之後，來強調這種目的。
to learn a foreign language well, you must make painstaking efforts.
mother saved every cent she could spare to pay for my schooling.
we must develop science and technology at high speed so as to raise scientific and cultural level of our country.
insgroupsto get a high mark in band 4, he did a lot of exercises both in grammar and reading comprehension.
b)不定式表示結果，特別是在so...as to, such...as to, only to...以及too...to等結構中的不定式表示結果。
i rushed to the station as fast as i could, only to find the train already gone.
she left her hometown with her beloved man, never to return.no one is too old to learn.
would you be so kind as to carry the luggage for me?
c)不定式常用來修飾形容詞，構成下列詞組：be able to, be afraid to, be apt to, be bound to, be certain to, be easy to, be eager to, be fit to, be likely to, be ready to, be sure to, be unable to, be unwilling to, be willing to等。
chinese team is bound to win the world cup.
it is likely to rain today, for it is very cloudy.
i am willing to help you with your homework, for we are friends.
6)不定式作賓語補語不定式常跟在下列動詞之後作賓語補語：ask, advise, allow, beg, cause, compel, command, enable, encourage, expect, feel, force, find, hear, have, inform, invite, let, make, mean, notice, order, permit, persuade, remind, require, request, teach, tell, urge, watch, warn, watch等。
because of the complexity of the modern equipments, most offices require secretaries to have specified training.
he asked you to call him at ten o’clock.
the note reminds me to be careful whatever i do.
i’d never allow my children to behave like that.
當不定式在let, make, have, hear, look at, listen to, feel, observe, watch, notice, perceive(感覺到)等動詞後面作賓語補語時，不定式不帶to.
whenever something is wrong with you, please do let me know.
i will have the students write a passage about internet.
i saw my mother shed tears at the news that the neighbor girl got seriously hurt in a car accident.
it seemed so long before he heard the stone hit the water.
7)不定式作主補帶有賓語及賓語補足語結構的動詞變為被動語態時，原來的賓語變為主語，賓補則變為主補。加主補的動詞主要有：assume, believe, know, report, say, suppose等。
mr. brown is said to have left for italy last week.(it is said that mr. brown left for italy last week.)
persons have been said to climb on roofs, solve mathematical problems, compose music, walk through windows and commit murder in their sleep.
he is reported to have won the 100瞞eter running race in the olympic games.
8)不定式作定語
a)不定式作定語通常要放在其所修飾的名詞或代詞之後。
不定式常作下列的名詞定語：attempt, ability, anything, chance, desire, determination, decision, effort, failure, intention, need, opportunity, plan, promise, pressure, right, tendency, time, way等。
his efforts to carry out the plan were successful.i have no intention to go to the cinema with you.
there is no need to bother him with such trifles.
there is a tendency to writer quite long sentences in commercial correspondence.
b)由only, last, next,序數詞或形容詞最高級修飾的名詞常用不定式作定語。
mr. zhang is always the first man to arrive at the office and the last man to leave.i don’t think he is the best one to do the work.
9)不帶to的不定式在下列詞組後面的不定式不帶to：would rather ...than(寧願……也不), had better...(最好)，can’t help but...(不得不), had rather...(寧願)，cannot but...(不得不，必然)，may/might as well...(不妨)，let alone(更不用說)。
you’d better return the books to the library on time. otherwise, you will be fined.
i can’t help but wish that nothing would go wrong.
they had never seen such delicious food, let alone eat it.