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Hi,

Let us take a look at the cases involving the past perfect progressive.

John had been playing tennis for ten hours before his mom showed up.

Here, you can say John has been playing tennis for ten hours until his mom showed up and that whole scenario has occurred in the past and we don't know whether he has kept playing tennis after his mom has showed up or not. He might as well as have played for two hundred hours and still keep playing as the talk is going on.

What I am not sure is whether he had stopped playing for sure after his mom showed up
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Here, you can say John has been playing tennis for ten hours until his mom showed up and that whole scenario has occurred in the past and we don't know whether he has kept playing tennis after his mom has showed up or not. Only this is meant in this case.

He might as well as have played for two hundred hours and still keep playing as the talk is going on. No, for precludes that, it means for a duration of