Ive been writing letters for an hour.
Ive been sitting in the garden.
2)过去完成进行时: 过去某个时刻以前一直在进行的动作
Id been working for some time when he called.
We had been waiting for her for two hours by the time she came.
3)将来完成进行时: 将来某个时刻以前一直在进行的动作.
By next summer, he will have been working here for twenty years.
In another months time shell have been studying here for three years.
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