This post was archived from Duolingo forum.
Credits to kf_b for the original post.
First let me link you some videos that have helped me, then I'll offer my own explanation. Definitely recommend going through these even though they look lengthy; they are worth it.
I'm gonna assume you already know that 는/은 means "topic marker" and 이/가 means "subject marker". Here's some key things that have helped me understand them.
- 는 can be used to make generalized statements.
- 사람들은 바쁩니다. (People (in general) are busy.)
- 는 is used to establish a new topic of discussion or change it.
So, if you hadn't been talking about anything, tack on a 는 to establish the topic. Or, if you had been talking about, say, movies, if you were previously talking about The Avengers, but now you want to talk about Jurassic World, you would add a 는 there. Make sense? Also remember that in everyday speech, once the topic has been established, the subject is often omitted because it is very obvious! For example, if the topic is about 날씨는 (weather), you wouldn't have to keep saying 날씨가 every time, because it is obvious you're talking about the weather! See the function?
- 는 is used to contrast something that is being said from something previously said.
- 이 커피는 좋아요. 그 커피는 싫어요.
So the 는 on the first clause sets the topic as talking about "this coffee". But the 는 on the second clause changes the topic to something contrasting (liking vs not liking the coffee). 는 can be sometimes translated, if it helps you understand better, to mean "As for...", or "Regarding xyz", etc. "This coffee is good. (As for that coffee however) That coffee is bad."
Also noteworthy: when a group of people are introducing themselves, they'll each start off by saying "저는", right? Well, they do this because they're changing the topic from talking about someone else to talking about themselves. 저는 잭입니다. "(Yeah the previous guy's name was Bob but now the topic is on me and as for me), I am Jack."
- 가 is used to establish and highlight a central subject in the sentence, putting the focus on the subject. Here's some sentences that might be said in a conversation.
- 오늘은 비가 왔어요. (The topic is about today and what happened today, and today it rained. But the fact that it rained was more important than the fact that today is today, hence the 가.)
- 제가 먹었어요. (The topic is still about what happened today, but the fact that I ate something was more important.)
So the 가 is used for something that exists within the realm of the topic and is something that is highlighted because it is important to the sentence. Whatever has a 가 attached means it is more directly connected to the verb (hence "subject-verb" and not "topic-verb").
I remember my confusion between the two particles when I was learning it stemmed from the fact that sometimes it was ambiguous why a sentence put 가 and another put 는. The answer I've found depends on 1) context, and 2) sometimes the subject and the topic can overlap. Sometimes the subject IS the topic.
Anyway, I hope that kinda helped. What helped me the most is kind of forgetting about it, accepting that it's confusing, and just exposing myself to different sentences and conversations. The more you see when Koreans use what particle, the better you'll get at understanding them. And to be honest, I wish Duolingo was better at teaching the particles; if they could implement a feature that gave you context or backstory for a certain phrase or sentence, which would mean you would have to figure out whether 가 or 는 was more appropriate. If you have more questions or want to talk about more examples, let me know!