eg:To see is to believe. (主、表)
It only took us a year to finish the work. (主，it形式主语)
She wants to play basketball this afternoon. (宾)
She was the first person to think of the idea. (定:主谓关系)
I have nothing to say on the problem. (定:动宾关系)
He asked me to help him. (宾补)
We are all happy to hear of the good news.(状)
◆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。
eg: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.
We felt the house shake.
Someone was heard to come up the stairs.
eg: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.
eg:There is nothing to worry about.
I need a pen to write with.
Give me some paper to write on.
eg:It is important for us to master English.
It will be a mistake for us not to help them.
I open the door for her to come in.
3.不定式的否定式:not to do
不定式的被动式:to be done
不定式的完成式:to have done(不定式的完成式仅强调其动作发生在谓语动词之前)