Quote Originally Posted by it-ogo View Post

Они долго ходили по магазинам.

Actually, I don't understand why past perfect is needed here. It is past and it means that action is over anyway. Why don't you say just "They shopped for a long time?"
These are two different things. Without any context you wouldn't usually say "they had been shopping for a long time". The past perfect tense can only be used in relation to a point in time you are talking about. Anything prior to that point is past perfect. English sees it fit to distinguish between something which is past in relation to the present, and something which is past in relation to a point in time which is also in the past. Yes, this is actually helpful.

So, if I see a sentence like "They had been shopping for a long time" I immediately know that the speaker is actually talking about a point in time which lies after the whole shopping event, but still in the past. I know to expect something like "and when they came home (which is the moment he is actually talking about) they were utterly exhausted".

Russian does a similar thing using adverbial participles like this: сделав уроки, он отдыхал. The adverbial participle tells you that one action is in the past of the other action. It's not as versatile as having a full-fledged tense because it can only apply to the subject of a connected expression.

As for the original question of the thread, it is important not to mistake the English distinction of progressive vs. simple tenses for the same thing as the distinction of perfective and imperfective aspects in Russian. They share some similarities, but they are not the same thing. Grammatical features present in one language do not automatically have a counterpart in another language, just as words cannot always be translated 1:1. I'm coming from a native language (German) which has neither progressive nor perfective aspects and still works well. We use other means, words rather than grammar, to mark something as continuing or finished.

Different features in different languages engender different ways of thinking. Learning a language is mostly trying to come to terms with these different ways of thinking and expressing thought, not about mere translation.