Questions & Answers about Что они покупали в магазине?
Why is покупали translated as "were buying" instead of just "bought"?
покупали is an imperfective verb. In the past tense, imperfective verbs often describe an ongoing process, which is why we translate it as "were buying". If we wanted to ask about the completed result ("What did they buy?"), we would use the perfective verb купили instead.
Could I use the perfective verb купили in this exact same sentence?
Yes, but it changes the meaning! Что они купили в магазине? means "What did they buy in the store?" By using the imperfective покупали, the speaker is showing interest in the process of shopping itself, rather than just the final list of purchased items.
Why does the verb end in -ли here?
In the past tense, Russian verbs don't change based on the person (I, you, we). Instead, they change based on gender and number. Since the subject is они (they), we use the plural past tense ending -ли.
Why is it в магазине and not just в магазин?
This is the Prepositional case, which you use when describing where an action happens. Because the buying happened inside the store, the preposition в (in) forces the masculine noun магазин to take the ending -е.