Hi Kizhi, I'm not a korean native speaker, it could be a very simplified explanation, but this is what I know: 는/은 = Topic marker, usually it is used at the beginning of a new thought, topic or theme. Think of it as the first sentence in a paragraph. E.g - 나는 켜요, 저는 피곤해 느껴요, 당신의 머리는 짧아요, 저는 스무한 살입니다. 이/가 = This is what we would call the 'subject'. Generally you can use this whenever you have a subject that is interacting with an object (를/을). There is alot of ambiguity about when to use 는/은 vs. 이/가, generally, you use 이/가 when your adding or subtracting information from the 'topic sentence'. E.g. 오늘은 날씨가 추워요 = Today (topic) the weather (subject) it was cold (verb). 저는 그 여자가 아주 예쁜 것 같았지만 그다음에 여자가 돌어쎠는데 여동새이야!!! (I thought (topic) that girl (subject) was very beautiful, but then she (subject again) turned around, it was my younger sister!). 를/을 = is actually the easiest to remember and use (also, many Koreans remove 를/을when speaking, especially if the context is understood). You use this whenever you want to indicate that something is doing something (i.e the direct object): E.g = 저는 먹었어요 = literally = I ate. "But what did you eat" = object, 저는 김치를 먹었어요. I ate (verb) kimchi (object). 나는 한국어를 배우고 있어요. 제 아내는 돈을 받아줬어요 (My wife was given (verb) money (object)). Hopefully this helps you, i actually am struggling to come up with examples, I'm blanking LOL!

I am dying on his example here. It is so hilarious. you can read his answer. It's very helpful. I am sure. I am going to give you more example concerning with your example. 나는 가방이 있다. I have a bag/I have the bag 나는 그 가방을 좋아한다. I like the bag 나는 내 가방을 들고있다. I am holding my bag 내 가방은 거기 있다. There is my bag 내 가방이 거기 있다. There is my bag 내 가방은 노란색이다. My bag is yellow well, my example is no good. Sorry.