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?