4 A: How many books ........ (you/buy)? B: I only ........ (buy) one.

A: How many books did you buy? B: I only bought one. **Translation:** A: Сколько книг ты купил? B: Я купил только одну. **Explanation:** In the first question, we need to form a past simple question using "you/buy". The correct past simple form of "buy" is "did you buy". Therefore, the question is "How many books did you buy?". In the second sentence, we need the past simple form of "buy". The correct past simple form of "buy" is "bought".