动词不定式在英语中的使用频率很高，可以作主语、表语、宾语、同位语、补语及独立成分等。带to的不定式是经常见到的，但有时也会遇到省略to的不定式。那么，什么时候可以省略to，什么时候又必须保留呢？这是大家学习不定式时的难点，下面笔者就常见的不定式的几种用法进行一下简单总结。
一、 不带to的不定式常见于以下几种情况：
1. 如果主语是all或是what引导的从句，主语受first, only, one, least或形容词最高级修饰, 且后面的从句或短语中有实意动词do时，作表语的不定式前不用to。如:
The only thing I can do was keep silent.
What he wants to do is complete the whole task as soon as possible.
All he does is study hard.
all作主语时，如果从句含有say，其后作表语的动词不定式有时可以省略to。如：
All they say is hurry up.
但在其他情况下，to不能省略。如：
All I wanted was to help him.
2. 在一些固定搭配中，如：let go，make believe，go hang等，不用to。如：
We’ll have to make do with dry bread.
He made believe he was innocent.
They can’t let things go hang.
3. 在would rather，would sooner（宁愿……也不）， might (just) as well，cannot help but，cannot but (不得不)，do nothing besides，do nothing but，do nothing than，had better，had best等结构后，用不带to的不定式。
The soldiers would rather die than surrender.
I cannot but decline his offer.
He did nothing else than cry.
4. rather than，sooner than (宁愿……也不)位于句首时，其后的动词不用to。
Sooner than run the risk of losing everything，they aGREed to the terms.
当rather than，sooner than位于其它位置时，其后的不定式可带to，也可不带to。如：
He prefers to rent a car rather than to have one of his own.
Mary decided to write rather than telephone.
5. 在feel, hear, listen to, see, look at, watch, observe, perceive, let, make, have等动词后，动词不定式作宾语补足语时不用to。如：
I have heard both teachers and students speak well of him.
He made me give up the exam.
6. 为避免重复,不定式可省去to。如：
He was at a loss as to what to think and say. (say前省略to)
It is just impossible to see that and not feel angry.(feel前省略to)
但是如果两个不定式表示对照或对比，则不可省略to。如：
To die or not to die—that is the question.
They came not to save us, but to conquer us.
7. 在介词except, than, besides, save, but 之后，如果这些词前有动词do，则常用不带to的不定式。如：
What do you like to do besides swim?
She can do everything except cook.
We have nothing to do save send for the doctor.
但当动词不是do 时，to不能省略。如：
The doctor told him nothing but to stop smoking.
8. why+不带to的动词不定式，引导疑问句，表明做某事是愚蠢的或无意义的，含有不满或批评的语气。
Why leave the door open?
why not+不带to的动词不定式，可用来提出建议或劝告。
Why not take a holiday?
Why not answer my question?
