同学你好，该知识点来自沪江网校《英语零基础直达大学六级【8月班】》的课程，想要更系统的学习，欢迎进入课程学习。不仅可以和更多的同学一起学习，而且还有老师、助教随时的学习指导和知识点解答哦。

同学你好~
1.作主语
可以直接作主语。这时候不定式在谓语的前面。如：
To see is to believe.
但在英语中，常用it作形式主语，真正的主语即动词不定式放在后面。如：
It's wrong to play tricks on other people.
It's our duty to keep our environment clean and tidy.

2.作宾语
want，decide ，agree等动词后面跟不定式。如果不定式作宾语，说明的是动作的内容。如：
We agreed to start early.我们同意早点动身。
She wants to be a doctor.她想当医生。

3.作宾语补足语
tell，ask，want，order，teach，invite，warn，wish，help，get等词后面常接不定式作宾补。如：
I tell him not to go there by bus.
我告诉他不要坐公共汽车去那里。
Edison's mother taught him to read and write.
爱迪生的母亲教他读书写字。

4.作定语：放在被修饰的名词、代词后面
I have a lot of work to do.
我有很多工作要做。
The doctor said he could do nothing to help the boy.
医生说他对这个男孩无能为力。

5.表语：放在连系动词be后面
His wish is to become a scientist.

6.作状语
a.目的状语：放在go，come，use，live，in order等词后面。如：
I come to see you.
He runs fast in order to get there in time.
b.原因状语：放在sorry，glad，surprised， disappointed，excited等词后面。如：
I am glad to see you here.
I am sorry to trouble you.

You need to practice what you have learned.  
这里“需要做……”，“做……”是“需要”的内容，是“需要”的宾语，因此这里to practice是need的宾语。

祝同学学习进步~