1.使役动词let，make，have 及感官动词后的不定式作宾语补足语时要省略to
His appearance made me laugh.他的样子使我发笑。
I saw the boy fall from the tree.我看见这个男孩从树上掉下来。
let fly 发射 let go 放开 make believe 假装 make do 凑合着用make, have, let, hear, see, watch, notice, help等后面的宾语补足语可以省略to的动词不定式：
The police had him describe what his robber looked like.
警察让他描述抢劫犯的样子。
We know what to do to make it work.
我们不知道如何让它起作用。
He stepped aside to let me pass.
他走到一边让我过去。
I saw him head out Jackson's house.
我看见他从杰克逊家出来。
I often heard the girls sing in the music classroom.
我经常听到女孩们在音乐教室里唱歌。
see, watch, notice后也可能用现在分词doing或过去分词done做宾语补足语。也就是其被动语态要带上to。
I was made to wait two hours before I was examined by a doctor.
我等了两个小时医生才给我检查。
She was seen to enter the building about the time the crime was committed.
有人看见她大约在案发时进入大楼。
2.并列不定式中第二个以及后面的动词不定式均省略to。即and, or, except, but, than连接两个并列动词不定式,后面的要一般省略to：
I wrote this letter to persuade and （to）encourage Mary.
我写这封信是为说服并鼓励玛丽。
I don’t know whether to stay or （to）leave.我不知道是留还是走。
I would rather do some reading at home than go shopping with them.
我宁愿在家看书也不愿和他们一起去购物。
I cannot promise anything but think of this week as a trial period.
我不能保证什么，只能把这周当作试用期。
It's easier to get into debt than (to) get out of it again!
欠债容易还债难。
3.句子中有动词do的某种形式时，but （或except）后的不定式省略to
do nothing but do sth.只好做某事
have nothing to do but do sth.只能做某事
there’s nothing to do but 只好做某事
I could do nothing except agree.我除了同意，没有别的办法。
He never did anything but watch TV.除了看电视，他从不干任何事。
He has nothing to do but drink.他无事可干，只有饮酒。
There is nothing to do but wait for the results.除了等结果我们什么都做不了。
She can do everything except cook.除了做饭之外她什么都会。
There’s little we can do except wait.除了等待我们没有什么办法。
4.主语中有动词do的某种形式时，作表语的不定式常省略to（句子中有动词do，解释它的具体意思的动词不定式可以省略to）：
What I really want to do is （to）go to the cinema.我想做的是去看场电影。
The only thing to do now is （to）go on.前进是现在唯一的出路。
What you first do is （to）mix the egg with flour.你先得把鸡蛋和面粉和好。
The only thing I can do is (to) keep quiet and sit this one out.
我唯一能做的就是保持沉默，等待这件事结束。
If he was prepared to apologize, the least she could do was (to) meet him halfway and accept some of the blame.
如果他准备道歉，她至少可以做个让步，承认自己的一些不当之处。
5.特殊句式中不定式的省略
cannot but do sth.只好做某事 cannot choose but do sth.不得不做某事
can’t help but do sth.只好做某事 had better do sth.最好做某事
Why （not）do sth.？ 为什么（不）做某事？ would rather do sth.宁愿做某事
I couldn’t but choose to wait.我只能选择等待。
He could not choose but leave her children at home.他只好把她的孩子们留在家里。
Why go with him？ 为什么要同他一起去？
Why not try to expand your story into a novel？ 你怎么不把你的故事扩展成小说呢？
6.只保留不定式符号to的情况
（1）某些动词后接不定式作宾语时的省略形式只保留to。
afford care decide forget hope intend like love mean need prefer.
refuse seem try want would like would love
You can do it this way if you like to.如果你愿意，你可以这么做。
I shall go if I want to.如果我想去就去。
I have asked her to come，but she does not care to.我让她来，但她不想来。
You may go if you wish to.如果你想走就可以走。
— Are you and Mary getting married？ 你和玛丽快结婚了吗？
— We hope to.我们希望如此。
（2）某些动词后接不定式作宾语补足语时的省略形式只保留to。
advise allow ask expect forbid force order permit persuade tell warn wish.
The boy wanted to ride his bicycle in the street，but his mother told him not to.
这个男孩想在街上骑自行车，他母亲告诉他不要骑。
Don’t go till I tell you to.等我叫你走你再走。
She wants to come but her parents won’t allow her to.她想来，但父母不允许。
If he doesn’t want to go there，you can’t force him to.若他不想去，你不能强迫他去。
He didn’t come though we had invited him to.尽管我们邀请他来，但他没有来。
（3）某些词组后也可承前省略不定式只保留to。
ought to have to used to be able to be going to
Though you do not want to ask him again，I am afraid you will have to.
尽管你不想再去问他，但恐怕你不得不去。
— Should I start at once？ 我应该立刻开始吗？
— Yes，you ought to.是的，你应该立刻开始。
（4）某些形容词后的不定式的省略形式只保留to。
afraid ready happy willing glad pleased delighted
They shouted to him to jump into the bus，but he was afraid to.
他们叫喊着要他跳上公共汽车，但他不敢跳。
— Could you mind looking after my cat？ 你能帮我照看一下猫吗？
— Not at all.I would be happy to.我很愿意。
【提示】
①如果不定式中含有be，have done时，要保留be，have。
He is not the man he used to be.他已不是原来的那个样子了。
He hasn’t finished his job yet.Well，he ought to have.他没有完成任务。但他应该完成。
②如果不定式是完成时的被动语态，要保留have been。
— Did he tell you to attend the meeting？ 他告诉你参加会议了吗？
— No，I ought to have been.没有，我本应该被告知。
最后常常用and代替to：
英语口语里，谓语动词后的动词不定式，常用and代替to（尤其是come和go后）：
Can Anne come and play?
安妮能来玩吗？
Come and see us if you're at a loose end.
有空的时候到我们这儿来坐坐。
What time will the taxi come and pick us up tomorrow?
明天出租车什么时候来接我们？
Go and buy yourself an ice cream.
去给自己买个冰淇淋。
Would you like to go and see a film tonight?
今晚你想去看电影吗？