안녕하세요! Good afternoon Let me try to help you understand between this two particles. I can not exactly explain in terms of grammar, but i will tell you how it works in a different way. 배아픈테 약은먹었어요?= You're sick, so did you take medicine? The 약은 part can be translated to something like "what about medicine? Did you take it?" Also, in this case, you can ask without 은 which means.. 배아픈데 약먹었어요? is correct as well. However, 배아픈데 약을먹었어요? could sound awkward. That is because it is translated as You have a stomachache, but you took medicine? So this sentence would not really correct to be used. When it more makes sense would be 배아픈데 아이스크림을먹었어요? You have a stomachache, but why did you eat icecream? Hope this helps :D