"Why are you eating soap?"
Translation:비누는 왜 먹고 있니?
Actually, this sentence is good for teaching usage of the particle "는". The best translation here uses "는" rather than "를"; not only the subject but also the object can become the topic. What's implied in this sentence is that you're not supposed to eat soap, or at least eating soap is strange or unexpected.
It was not the grammar that I was concerned with but thanks for the elaborated explanations. I just think this is a hilarious statement.
Actually it would make sense, but does not sound natural. It should be 왜 비누를 먹고 있어? if you bring 왜 to the front