If I want to emphasize both the process and duration of some action or event which was before another one, I should opt out for the past perfect continuous tense: "I had been studying for 3 hours before Mom came home" (UPDATE: I was told that in certain contexts, "before" essentially means "when")
Is it equally correct English to simplify the past perfect continuous to the past continuous (The way you do with the past perfect: "We left before you came home" instead of "We had left before you came home"):
"I was studying for 3 hours before Mom came home" instead of "I had been studying for 3 hours before Mom came home"?
I understand that in informal speech all kinds of things may happen, but I'd like to know the best practices which would be good for some formal exam.
UPDATE: The way I see it now, you should use the past perfect continuous with "before" when this conjunction means "when" (I'm still getting used to this apparently illogical thing).
But, e.g., the sentences "Marilyn was a factory worker before she was a model" or "I was dissembling before you were born" don't need perfect tenses because "before" in them means "before" and the succession of actions or events is expressed.
Someone told me that it was OK to say: "They were married for 10 years before they got divorced" or "They had been married for 10 years when they got divorced". Does it mean that I can substitute "when" with "before" in the latter example and that it would mean "when"? ("They had been married for 10 years before (meaning "when") they got divorced")