Here is what I think, 1. When introducing a new topic, subject markers are preferred. e.g. There is an apple. 사과*가* 하나 있어요. not 사과*는*... The apple is red. 그 사과*는* 빨개요. not 그 사과*가*... 2. When talking about something usual or general, (subject markers) e.g. The apple is red. 그 사과는 빨개요. (We all think like that, because apples are usually red) The apple is red. 그 사과가 빨개요. (I expected the apple is blue beause all other apples are blue, but somehow it's red, so I'm surprised) 3. When talking about "activity", (topic markers) e.g. I did = 제가 했어요.(not you, we, they but "I", talking about "who") I did = 저는 했어요.(talking about what I did because we know "I" did, but we are more interested in WHAT I did) There are some more, but it will be enough for introducing the differences betweem them. :) \^o^/