Мы употребляем do и don't для того, чтобы построить вопросительные и отрицательные предложения в простом настоящем времени:

• A: do you know Peter? B: Yes. We are old friends.  
• A: Do you like this music? B: Yes. It's great.  
• A: Do they live here? B: No. They live next door.
  
• A: What's that? B: I don't know.  
• A: Do they enjoy the theater? B: No. They don't go out very often.