（一）动词不定式用来作主语
To persevere means victory!
坚持就是胜利！
To see once is better than to hear a hundred times.
百闻不如一见。
但在很多情况下，特别是在口语中，我们常常用代词it来代替它作主语，而把它移到句子后部去，这样使句子显得比较平稳：
It is not an easy thing to master a language.
学好一种语言是不容易的。
It only took (us) a year to complete the project.
完成这项工程只花了（我们）一年时间。
如果要说明不定式表示的动作是谁做的，可以在不定式前加一个for引起的短语：
It is not hard for one to do a bit of good.
一个人做点好事并不难。
It will be a mistake for us not to help them.
我们不帮助他们是错误的。
（二）动词不定式用来作定语
I want to get something to read during the vacation.
我想找点书假期看。
Do you have anything more to say?
你还有什么话要说吗？
She was the first person to think of the idea.
她是第一个想到这个主意的。
There is nothing (for us) to worry about.
没有什么值得发愁的。
（三）动词不定式用来作宾语
Heofferedtohelpus.
他提出要帮助我们。
Ididnotthink(expect)tofindyouhere.
没想到在这儿碰到你。
Ihopeyou‘lladvisemewhattodo.
我希望你给我出主意怎么办好。
Noonecouldtellmewheretogetthebook.
没有人能告诉我哪里能找到这本书。
Wefounditimpossibletogeteverythingreadyintime.
我们发现不可能把一切都按时准备好。
Shethoughtitunneccessarytoarguewithhimaboutit.
她觉得没有必要和他辩论。
（四）动词不定式用来作宾语补足语
He wants you to call him back at 11.
他要你十一点给他回电话。
Would you like us to go with you?
你愿意我们和你一道去吗？
注意：在下列动词后常用不带to的动词不定式作宾语补足语：see, watch, hear, feel, let, have, make, notice, observe, small等。
Don’t forget to have him come.
别忘了让他来。
Suddenly we felt the atmosphere grow tense.
突然我们感到气氛紧张了起来。
（五）动词不定式用来作状语
They ran over to welcome the delegates.
他们跑过去欢迎代表们。
To protect his comrades, he heroically laid down his lif.
为了保护同志们，他英勇地献出了自己的生命。
She decided to work harder in order (so as) to catch up with the others.
她决定加紧学习好赶上别人。