Page 432 - Practical English Usage 3ed - Michael Swan, Oxford

past verb form with present or future meaning 426
past perfect progressive: actions or situations which had continued up to the past moment that we are thinking about, or shortly before it
[ went to the doctor because [had been sleeping badly.
3 how long? past perfect progressive. not past progressive
We use a past perfect progressive, not a past progressive, to say how long something had been happening up to a past moment.
We'd been walking since sunrise, and we were hungry. (NOT We were waHeing since sunrise ...J
When she arrived, she had been travellingfor twenty hours. (NOT • • • site ttIttS tffllfeUillg ...)
For the difference between since and for, see 208.
4 progressive and simple: differences
Progressive forms are mostly used to talk about more temporary actions and situations. When we talk about longer-lasting or permanent situations we prefer simple forms. Compare:
My legs were stiff because [ had been standing still for a long time.
The tree that blew down had stood there for 300 years.
Progressive forms generally emphasise the continuation of an activity; we use simple tenses to emphasise the idea of completion. Compare:
[had been readingsciencefiction, and my mind wasfull ofstrange images.
[ had read all my magazines, and was beginning to get bored.
Some verbs are not normally used in progressive forms (see 471), even if the meaning is one for which a progressive form would be more suitable.
[ hadn't known her for very long when we got married. (NOT .' htttln't been kn6fiJillg her ...)
426 past verb form with present or future meaning
A past tense does not always have a past meaning. In some kinds of sentence we can use verbs like [ had, you went or [ was wondering to talk about the present or future.
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