第一个情况是.用不定式表将来的目的性行为.
比如.
I borrowed some books to read during my holiday.
第二个情况是.用不定式修饰被序数词,最高级,no,any,all所限定的中心词.
比如.
He was the best man to do the job.
第三个情况是.用不定式修饰抽象名词.(像.ability,chance,idea,excuse,promise,等等.)
比如.
Do you have the ability to read and write English?