Once I tried explaining this to a friend of mine and failed miserably. But I'm nothing if not persistent, so here we go. There are some times, even as a native speaker, or maybe because I am, I have hard time explaining why we use 은/는 in some cases and 이/가 in some cases. But there definately are some differences. Susemi99's point may valid in some instances, but I'd say it's an over-simplification. And before I start, I have no idea as to where the name 'topic marking particle' came from. Koreans learn that 은/는 is the 한정,대조 보조사, meaning 'limiting or contrasting auxiliary particle.' (은/는 is not a 격조사(case particle), but a 보조사(auxiliary particle). This is a very important difference and it seems the English translation of the word doesn't really show the difference.. But I'm getting out of the topic.) Basically, 은/는 is an auxiliary particle that makes limitations or contractions. Having this standard is helpful in many cases. Using 은/는 for nouns that have already appeared in a speech can be explained with this and also so many other instances. >Examples 한 아이가 있었다. 그 아이는 키가 작았다. 어느날 아이는 산책을 나섰다. There was a kid. The kid was short. One day, the kid went out for a walk. In the last sentence, '아이는 산책을 나섰다', 아이 here is the one the speaker mentioned earlier even without the definite article. 꽃은 피었다. (flower(s) bloomed) / 꽃이 피었다. (flower(s) bloomed) The latter one merely means that flowers bloomed, while the former one implys that something else didn't bloom, or didn't go very well (this depends on the context really, but usually blooming of flowers is a good thing, so maybe something bad happened?) 나는 밥을 먹을게 너는 라면을 먹어라. (I will eat rice, you eat noodles) A contrast between 나 and 너 is being made. I-eat rice, you-eat noodes. 내가 할게/ 나는 할게 (I'll do it) The former one means that 'I' will do the work. The subject of doing the work will be 'I' is what the former one means. The latter one, however means something like 'I don't know about anyone else, but 'I' am going to do it.' See the difference? The former one is marking the subject while the latter one implies that, while the subject of the doing does not necessarily have to be 'I'(it can be 'we'), in contrast to those who won't do the doing, I will do the thing. 사람이 달린다. / 사람은 달린다. The former means that a person runs (maybe the speaker is at a park and is seeing a person running). The latter one means a person(as a mankind) runs. Unlike other species that do not run, people run. However this does not mean that all the other species do not run while only mankind runs. The usage would be like.. 새는 날고 사람은 달린다, contrasting birds and men. If you say 새가 날고 사람이 달린다, it sounds more like you are describing what you are seeing, not the general nature of the topic.

I'm going to say the same as susemi99 and Lee, but as I'm a learner too I guess it's easier for me to understand your doubt and explain it. If you say "I'll do it" Do you want to emphasize the subject? (The one who will do it is me. Not anyone else.) So you say 나는 할게. Do you want to emphasize the predicate? (I'll do it. Not anything else.) So you say 내가 할게. Example with a situation: Your friend is trying to open a bottle. You say "Let me help you. I'll open that for you." Saying this, you are emphasizing the subject. You want him to know who will open the bottle, not what you are going to do. Lee's first example is really good: 한 아이가 있었다. 그 아이는 키가 작았다. There was a kid. That kid was short. In the second sentence you already know the subject (the kid), so you don't need to emphansize it. The predicate gets more value because it's a new information.

Where is the Point. Subject has point, use '이/가'. Predicate has point, use '은/는'

I wanted to ask this same question but i saw yours. Korean markers really confuse me. I hope from your question comes good answers. So I can get an idea