It's past simple, because that action (spending 8 hours) comes after the previous actions (leaving work early & asking her friend)... _If the 3rd action (spending) happened BEFORE the other two, then would be "Had spent" (past perfect)

"Mary loves to shop" is definitely present tense because this is like her all time hobby "She left work early today...." It should be past tense because she is talking about the things that had already happened before. "The two of them..."It depends on what kind of purpose you are talking about, if you want to focus on the point that it is a fact that they spent eight hours buying, then past tense would be good. For past perfect, it would be more like a "wow", they have already spent this long time! It doesn't necessarily means that they have finished, maybe they are still shopping. I hope you can understand~ If you want to know more about English, feel free to talk to me!

Thank you, Matteo! I'm trying to feel it, because rules are not easy to recall when speaking:)

Funny, this one is a little harder than it looks. I agree with the girl from China. It kind of depends of the meaning you wish to convey. Let's take a quick look. Mary loves to shop. (Conversational English) So she'd left work early today and asked her friend to meet her at the mall. The two of them spent eight hours buying dresses and shoes. (Optional and fine is also: The two of them had spent eight hours buying dresses and shoes.) If I was with another friend and reporting, say over lunch, what Mary and her friend had been doing, I'd say it that way. More technically, the indicative mood, reporting past events of what transpired. Now let's look at this example: Say I was with another friend and I had heard or observed somehow what Mary was doing, then another way of saying it arises. I see Mary and her friend are still there shopping, so I turn to my friend and say, "The two of them have spent eight hours buying dresses and shoes. The last, usually with a rising intonation, as if asking a question to show disbelief, I can say to indicate to my friend that they're still there after eight hours. Hope that helps. No worries. In conversational English there tends to be more flexibility in how we use our words. That's why we have so many jokes about "play on words."