概念：表示某一动作开始于过去某一时间，延续或重复地出现至今，还将延续至将来； We have been working on this project for over a month now. It has been raining for two hours. It seems oil has been leaking from this pipe for some time. 概念：相对过去的某个时刻来说对现在已经有直接影响并且还在进行的动作； She had been suffering from a bad cold when she took the exam. John had been asking me the same question for almost 6 times. I had been looking for it for days before I found it. 概念：表示动作从将来的某一时间开始一直延续到将来某一时间, 并可能继续延续下去； Vicky will have been working in this School for 12 years by the end of next year. The play is coming off in August. By then it will have been running for three months. By the time John retires, Jerry will have been teaching for 10 years. 概念：表示动作从过去某一时间开始一直延续到对于过去来说的将来某一时间，并可能继续下去； I heard that by May Vicky would have been working here for twelve years. She told me she would have been teaching in the university for 20 years by that summer. He said that by the end of the spring term he would have been studying English for three years.
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