一、不定式用作主语
To see is to believe. 眼见为实。
To talk with him is a great pleasure. 和他谈话是一件非常愉快的事。
To remember this is very important. 记住这一点很重要。
【注】为了避免头重脚轻，在许多情况下，通常都将作主语的不定式置于句子后部，而在句首使用的主语位置使用形式主语it：
It’s very important to remember this. 记住这一点很重要。
It’s a pity to leave so early. 这么早走太遗憾。
二、不定式用作表语
He seems to be ill. 他似乎有病。
Her wish is to be a teacher. 她的愿望就是当一名教师。
You are not to smoke in this room. 你不应在这个房间里吸烟。
【注】不定式作表语主要有三种情况，一是用于seem, appear, prove, turn out等连系动词之后的不定式(尤其是to be)，二是像My job is to sweep the floor(我的工作就是擦地板)这样的主语与表语“等价”的情形，三是表示想法、约定、义务、命令、可能性、命运等，再如：
We are to meet at the station at three. 我们约定3点钟在车站见面。(表约定)
How are we to convince him? 我们怎么能说服他呢? (表可能性)
You are to come when I call. 叫你来的时候，你必须来。(表命令)
三、不定式用作宾语
I can’t afford to buy a car. 我买不起汽车。
Remember to post the letter. 记住把信寄了。
I hope to be staying with you. 我希望和你住在一起。
【注】(1) 不定式不仅用作动词的宾语，还可用作个别介词(but, except)的宾语：
I had no choice but to wait. 除了等，我没有别的选择。
It had no effect except to make him angry. 除惹他生气外，没产生任何效果。
(2) 当作宾语的不定式后跟有宾语补足语时，通常要用形式宾语it代替不定式，并将真正的宾语不定式置于宾语补足语之后：
I find it difficult to learn Japanese well. 我感到学会日语不容易。
四、不定式用作宾语补足语
Who taught you to drive? 谁教你开车的?
He warned me not to touch it. 他警告我不要碰它。
What caused him to change his mind? 什么使他改变主意的?
五、不定式用作定语
I have a question to ask you. 我有一个问题要问你。
It is a pleasant thing to remember. 这是一件值得记住的愉快的事。
He is not a man to tell a lie. 他不是个说谎的人。
【注】有的名词(如way, chance, right等)后用作定语的不定式可换成of doing sth：
It’s the best way to do [of doing] it. 这是做此事最好的办法。
但是，以下名词后接不定式作定语时通常不能换成of doing sth：
attempt 试图 courage 勇气
decision 决定 effort 努力
fortune 运气 failure 失败
invitation 邀请 permission 允许
promise 允诺 wish 愿望
六、不定式用作状语
不定式作状语可以表示多种关系(如目的、结果、原因、条件等)：
I went to France to learn French. 我到法国去学法语。(表目的)
He returned home to find his wife waiting for him. 他回到家发现他的妻子在等他。(表结果)
You would make a great mistake to accept his offer. 你要是接受他的建议，你就犯了大错误。(表条件)
We are proud to be members of this team. 作为这个队的队员我们感到很自豪。(表原因)
He opened his mouth as if to say something. 他张开嘴，好像要说什么。(表方式))
