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Could you check the following sentences out?

Thank you.

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1. He was reading the book for an hour.

2. He was reading the book for an hour when his friend came in.

3. 2. He had been reading the book when his friend came in. (without "for an hour")
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The first one indicates how long an action lasted, so it'd be correct to say

he read the book for an hour.

The second one says that one action happens before the other action takes place, so "it's the past of the past· and the verb tense used to express this is "past perfect" (Continuous)

He'd been reading the book for an hour, when his friend came in.

The third one is correct even if you put "for an hour"

Hope I helped.
Comments  
Thanks, jossx.