I’ve been writing letters for an hour.
I’ve been sitting in the garden.
2)过去完成进行时(由had been + ing分词构成): 过去某个时刻以前一直在进行的动作
I’d been working for some time when he called.
We had been waiting for her for two hours by the time she came.
By next summer, he will have been working here for twenty years.
In another month’s time she’ll have been studying here for three years.
4)将来完成时(由shall/will have + 过去分词构成): 将来某时已发生的事.
I shall have finished this one before lunch.
They’ll have hit the year’s target by the end of October.