英语什么情况下用不定式

英语有好多情况都要用到不定式，首先在一些固定结构中，比如asksomebody to do something ,tell sb to do sth 等中；另外不定式可以用来做句子的主语，表语等,如To say something is easy.(主语）My task is to finish this job.（表语）不定式的用途比较广泛，应该在具体的语言实践中总结归纳从而达到灵活掌握的目的。

1.用在一些及物动词后，如 want to do fail to do decide to do manage to do plan to do 2。用在一些疑问词后可做主语，宾语，表语等。When to start is not decided.3 用在介词but后做介词宾语，如 I did nothing but watch TV yesterday./ We had no choice but to wait here.4。用在动词后也可在句首做状语。如 He came to see me./ To/In order to catch the early bus ,he got up early.5。用在系动词后做表语。如My dream is to be a scientist.6. 用在句首做主语, To be a singer is my dream.7.用在名词或代词后做后置定语， The best time to come to Beijing is autume.8.用在宾语后补充宾语做了什么，叫宾语补语 Our teacher tells us to read more in the morning .]

动词不定式形式，是用来表达想做，但尚未去做的事。 如： My plan is to write 3 books in a year. 我的计划是要在一年里写出三本书。（想写，还没写） He decided to help his father on the farm. 他决定在农场帮助他父亲。（想帮，尚未去做） To make a plan for such a great project is not an easy thing. 要为这么大个项目做计划，不是件容易的事。（想做计划，但尚未实施）]

http://baike.baidu.com/view/26732.html?wtp=tt#8_2 看一下吧，主要看那个不定式的语法功能]