(3)将不定式移到句首表示条件，如：
to hear the teacher better， you have to sit in the front. = if you are to hear the teacher better，……
(4)与形容词连用时，大多表示原因。这些形容词主要有：happy， kind， surprised， frightened， angry， shocked， glad， delighted， grieved，disappointed等。
They were surprised to be informed of the news. i‘m glad to hear this.
动词不定式的时态和语态
一般式 to do to be done
完成式 to have done to have been done
进行式 to be doing
完成进行式 to have been doing
1.如果不定式所表示的动作与主要谓语动词所表示的动作同时（或几乎同时）发生或发生在主要谓语之后，通常用不定式的一般形式。如：
I saw him go out.
I‘m glad to see you.
2.如果不定式所表示的动作发生在主要谓语动词或特定的时间之前，不定式就要用完成式。如：
I intended to have telephoned you， but i forgot to.
I should like to have bought a dictionary.
3.不定式进行式表示主要谓语动词所表示的动作发生时，不定式所表示的动作正在发生。如：
I am glad to be talking with you. she pretended to be listening attentively.
4.不定式完成进行时表示的是一个开始于过去某一时刻但一直延续到谓语动词发生的那一时刻的动作。如：
He was thought to have been working on the project for two years.
5.不定式的被动语态
当不定式的逻辑主语是动作的承受者时，这个不定式要用被动式，包括它的各种时态在内。如：
She doesn‘t like to be treated as a guest.
她不喜欢被当作客人对待。
They want their tv set to be fixed right now.