In the book "Advanced Grammar in Use", it states:
The past perfect continuous is mainly used in written texts and is less common in speech.
What is the reason for that and how do people use the past perfect continuous and past perfect properly?
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As a rule, you can generally expect the past perfect continuous to be used when expressing the duration of an action, or specific time period. For example:
He had been talking for 5 minutes
They had been running all morning
He had been working on the book for several years
The past perfect is generally used to express the completion of an action, (sometimes in relation to another event in the past):
Thomas had never tried gravy before he went to the restaurant
I had finished all the work
We hadn't remembered to save the document
Past perfect continuous is uncommon both in speech and in writing, and maybe more uncommon in speech. The reasons for that may be debated, and I can think of the following:
I'm not sure about #1 and #2, but I think #3 is likely a good reason. Of course there may be other explanations.
Here's a PPC phrase we encounter more in oral communication (well, kind of) than in written text: "It's been a hard day's night and I've been working like a dog". Would that sound unnatural in casual speech?