(一)不定式：to +动词原形
简单句中如果已经有一个动词做谓语，那么再有一个动词就要在这个动词前加to做不定式。不定式在句子中可以作主语、宾语、表语、宾语补足语、定语、状语等。
To see is to believe.(眼见为实)
有时用it做形式主语，真正的主语放在句子最后。
It only took us a year to finish the work.
Once you get into the habit of smoking， you'll find it extremely difficult to get out of it.(get into the habit 养成……习惯)
She likes to play basketball this afternoon.
The important thing is to save people.
Not to grasp firmly is not to grasp at all.(不定式的否定放在to的前面)
She was the first person to think of the idea. (think of 想起)
He is always the first to come and last to leave the office.
I have nothing to say on the problem.
They warned him not to let out the secret.(let out 泄露)
We are all happy to hear of the good news.
Let's hurry so as not to be late for the meeting. (so as to do sth. 以便为了做某事)
如果not不是放在to的前面，那么它不是否定不定式而是否定整个句子。
They didn't warn him to let out the secret.
They warn him not to let out the secret.
(一)不定式：to +动词原形
不定式在let， have， make (让， 使)， see， look at， notice， hear， listen to， feel 等后面作宾语补足语时to 应省略，在被动语态中to应保留。
The coach has the team practice hard every night in the past two years.
I saw a man come into the classroom and talk with the teacher.
Don't forget to have him come tonight.
We felt the house shake.
The house was felt to shake.
Someone was heard to come up the stairs.
在help 后， 不定式可以带to 也可以不带to.
Help me (to) get him to bed.
He help her lift the box.
He helped her to mount the bike.
She was helped to lift the box.
当不定式和其修饰的词有动宾关系时，动词如果是不及物动词，后面就要有必要加介词。
There is nothing to worry about.
Is this something to be ashamed of ?
I need a pen to write with.
Give me some paper to write on.
Let's find a room to put the things in.
