如果不定式的动作发生在主句的动作之前，不定式应该用完成时即to have done或to
have been done;如果不定式的动作和主句的动作同时发生，不定式就应该用进行时，即
to be doing，不定式进行时一般没有被动态;如果不定式动作发生在过去某一时间并且
即to do或to be done。我们不妨列表如下:
│体别 │主动态 │被动态 │
│一般形式 │to do │to be done │
│进行时 │to be doing │一般不用 │
│完成时 │to have done │to have been done │
│完成进行时│to have been doing│一般不用 │
They are said to have graduated from the University.
When he came in。I happened to be lying in bed.
Researchers are known to have been doing a vast amount of work during the past few
years in improving nat[一ral material.
The bank was reported to have been robbed yesterday.
某些表示愿望、想法的动词的过去时+to have done八o have been done，系动词be的
过去时was或were+to have done八o have been done，某些情态动词+have done/
have been done可以表示本来打算做而实际未完成的动作。例如:
We were to have met at ten.
I intended to have had the bike repaired last night，but I was too busy at that time.
They hoped to have completed the project before June.
You should have filled out the form earlier.
But for the delay I could have finished the work.
注意:这里所指的表示愿望、想法的动词过去时主要有:wished, expected, desired, in-
than，cannot，may as well，would just as soon，would rather…than，do nothing but
I would rather go later.
You had better do the assignment by yourself.
I would just as soon stay at home.
I would rather die than betray my motherland.
All I did was empty the bottle.
All she seems to do is gossip with her neighbors.
The worst you can do is become very lazy.
The best thing to do was wait and wait.
What I really wanted to do was drive all night.
2)在某些固定搭配中，如make do with, make believe, let slip, let go, leave go, hear
+动词原形)。即purpose, objective, goal, aim, function, reason, intention ...+is
+to do sth....。例如:
The primary reason why the constitution requires a census every ten years is to provide
a basis for the apportionment of representative among the states.
His aim is to become a congressman.
某些动词+介词+宾语结构后须接不定式作宾语补足语。如arrange for, ask for,
long for，wait for，vote for, count on，rely on等。例如:
I will arrange for you to come.
We waited for that to be done.
You can't depend upon john to be there when he is needed.
某些动词不能直接接不定式，须在不定式前加上疑问词，如know, teach, explain, tell
He didn't know whether to laugh or to cry when he heard the words.
He has learned how to write a business letter.
I hope you'll explain to me how to do the job.
He got there in a hurry only to find the train had gone.
He came into the manager's office only to be told that he was dismissed.
如果在。my to do前加上if,则表示原因，意为“仅仅因为”，亦可变成从句if only be-
Instructors may present their own views，if only to demonstrate that it is possible to
make carefully reasoned choices about ethical dilemmas.
There will be more famines，if only because the world's population is growing faster
than food production.