to do不定式可以做主语，宾语，表语，补语以及定语和状语。
1、做主语。
不定式做主语表示具体的一次性行为。
To finish the work is the park this morning.
2、做表语。
不定式做表语表示具体的一次性行为，也表示对主语进行解释。
The task this afternoon is to finish the work.
I'm to inform you that you are fired.
3、做宾语。
不定式做宾语，有很多动词搭配，需要平日的积累。举例有。 Refuse ， encourage， seem， offer， afford， pretend ，promise， decide.
4、做补语。
不定式作补语，有两种形式，动词原形或to do。比如：
see/watch/observe/notice sb do sth.
hear/listen to sb do sth
feel sb do sth
make sb do sth
have sb do sth
let sb do sth.
get sb to do
5、做定语
He is always the first to come and the last to leave.(序数词后用不定式)
He is the very/only one to do the job.( The very /the only后用不定时)
Is this the best way to help him?(比较级最高级后用不定时)
The next train to arrive is from Washington.（不定式做定语表将来）
There is something to be taken to your sister.（不定代词后面用不定式做定语）
He has kept his promise to write to his parents regularly.（抽象名词之后用不定式做定语）
6、做状语。
I'm sad to hear the bad news.（形容词之后做原因状语）
To catch the early bus， I have to get up early.（不定式表目的）
He speaks too fast for me to follow.
The child is not old enough to go to school.（ too或 enough后表示程度）
He hurried to the station， only to be told the train had left.（不定式表出乎意料的结果，前面可以用only或者just修饰）
