"The cat eats bread."
Translation:그 고양이가 빵을 먹습니다.
그 means "this". It's difficult to use 그 in a short sentence like this. Because there are three words to say "this": 이/그/처. There are rules for these words. You need somebody to talk when you use these words. You say 이 고야이 when you and the other person can reach the cat with the hands. 그 고야이 when the other person can reach the cat, but you don't. 처 고양이 when both of you can't reach the cat.
는 is a topic marker while 가 is a subject particle. When you use 는/은, it implies that there's a function of emphasis and/or contrast, unlike using the subject marker 가/이
It means, like, 가방은 예쁘다, that something else isn't as pretty as the bag.
So, 가방이 예쁘다, only this bag, that is around is pretty, no more to it, the end.