[Past Perfect Continuous Tense] We apologised because we had kept them waiting...

[Tofu]

Hello Alan, Mister Micawber, Beeesneees, Mordant, Esl_Expert and other native English speakers,

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http://www.learnenglish.de/grammar/tensepastperfcont.htm
We apologised because we had kept them waiting for 3 hours.
We apologised because we had kept them waiting since lunchtime.
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Can I say "kept" (simple past) instead of had kept?

[Beeesneees]

If you just want to tell what happened some time in the past you can use 'kept'.
If you want to tell what had happened before/up to a certain time in the past, as these sentences do in the current form, then you need to use 'had kept.

In these sentences as written, 'we' are looking back from a certain time in the past to tell what had happened before.
If you change the tense, you simply tell what happened in the past.
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Cheers m' dears!

[Tofu]

Thank you, B.