有些名词,如attempt, chance, effort, freedom, intention, necessity, opportunity, reason, time, way, etc. 其后既可跟不定式，也可用介词+ -ing分词。例如：

 They made an effort to finish the work in one day.

  They made an effort at finishing the work in one day.

 I'm glad to have the opportunity to speak to you.

  I'm glad to have the opportunity of speaking to you.