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How long had they been sleeping-when he woke them up?

The more correct sentence should be "How long had they been slept-when he woke them up?"

because they had ended their sleep after he woke them up.

My question was raised after reading the topic "When to use 'had been' + past participle of the verb"

Correct me if I am being wrong.

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closed as not a real question by tchrist, MετάEd, Andrew Leach, Kristina Lopez, Hellion Mar 1 '13 at 16:50

It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and cannot be reasonably answered in its current form. For help clarifying this question so that it can be reopened, visit the help center.If this question can be reworded to fit the rules in the help center, please edit the question.

Your premise is incorrect. "When he woke them up" they had to be sleeping or he wouldn't have been able to wake them up, by definition. In English you would never say "been slept". If you were to use slept in this sentence you could say: "How long had they slept when they were awakened?" The example from your linked question is not equivilent. In the case of a couple who are engaged to be married, the word "engaged" is not necessarily in past-tense. "We are engaged", "We will be engaged" and "We were engaged" are all equally valid English. – Joel Brown Mar 1 '13 at 13:21

Though this question will probably soon be closed as "off topic" or "not constructive", I feel the need to answer it as I hope these simple words of mine can clear OP's doubts regarding this seemingly confusing concept.

In short, past perfect progressive is the correct choice of tense after been.

How long had they been sleeping?

If been is not necessary to be used, you can consider having the verb in past perfect

How long had they slept?

which is slightly different from simple past.

How long did they sleep?

You should not use past perfect after been in this case because it would result in a type of grammatical construction called passive voice.

Not passive:

John had thrown the ball.

A sentence with the same meaning, but passive:

The ball had been thrown by John.

Note: next time when you have a similar question, ask it in ELL exchange. EL&U is not really the right community for it.

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