用完成(进行)时；现在或者过去完成（进行）时都有.视乎情况.[from这里相当于since.]
例如：
He has been doing homework from/since 6:30.(从六点半开始他一直在写作业.)
They had worked on that machine from 3:00 am and didn't stop until the next daybreak.(从凌晨三点开始他们一直在机上工作,直到第二天破晓.)
完成进行时是用来强调动作的一直持续的,具体请另行参考.
