Like many other Korean learners, I also have a hard time distinguishing 은/는 and 이/가. Then I researched on the internet about this problem and found a video about it. You can watch that video below:
Below this video there is a comment by a person named "Zoom In & Out", and this person shows how to distinguish 은/는 and 이/가 as follows (I copied it exactly so it was a bit long):
"These subject markers or particles in the Korean language may appear a bit confusing. However, it is actually extremely easy to use once you understand the difference between the two groups 은/는 and 이/가.
It's simply a way of emphasizing a part of speech in sentences. In English, you stress a word to deliver your message and subsequently change the meaning by placing a stress on a word, for instance, (The word in parenthesis is pronounced with a pitch),
(I) didn't say he stole the money. (Someone else said it.)
I (didn't) say he stole the money. (That's not true at all.)
I didn't (say) he stole the money. (I only suggested the possibility.)
I didn't say (he) stole the money. (I think someone else took it.)
I didn't say he (stole) the money. (Maybe he just borrowed it.)
I didn't say he stole (the) money. (but rather some other money.)
I didn't say he stole the (money). (He may have taken some jewelry.)
(Source: American Accent Training by Ann Cook, Chapter 24 P.185, 2nd Edition, 2000)
You can find the seven Korean translations at the end of this.
The Korean language uses different subject markers, 은/는 or 이/가 to emphasize a part of a speech, either predicate or subject. It's relatively simple compared to the way English speakers change the meaning of a sentence as demonstrated above:
If you want to emphasize WHO or subject, use 이 or 가. If you want to emphasize predicate (What, Where, When, How and Why), then use 은 or 는.
Now, look at the following examples:
Situation #1. A teacher comes in the classroom and finds out that it is nice and clean. The teacher asks students, "Who cleaned the classroom?"
Question: If it is 'YOU' who cleaned the classroom, not other student, what would you say?
a. 저는 교실 청소를 했어요. b. 제가 교실 청소를 했어요.
The answer is b. Although both are translated into "I cleaned the classroom" in English, the right subject particle that emphases the action doer is '가', meaning it was ME who cleaned the classroom. On the other hand, the the first sentence simply means, "I cleaned the classroom."
Situation #2. A teacher asked Mike when he turned in his homework assignment.
Question: What would you say if Mike replied that he turned in his assignment yesterday.
a. 저는 어제 숙제를 제출했어요. b. 제가 어제 숙제를 제출했어요.
The answer is 'a'.
Situation #3. Ms. Kim comes back home from her work and sees one of her two sons, 명수, watching TV in the living room. She asks 명수, "Where is 진수?"
Question: Which one is the right translation for "Where is 진수?"
a. 진수는 어디에 있니? b. 진수가 어디에 있니?
The correct answer is 'a' because the question is about WHERE (predicate), not WHO (subject).
So, if you want to emphasize WHO, use 이 or 가. All others, use 은 or 는!!
Here's some more examples:
A: Where is Tom going? (톰은 어디가니?) B: He is going to a bookstore. (톰은 서점에 가는 길이야.)
A: What time are you going to be home? (너는 언제 집에 오니?) B: I will be home by 7. (저는 7시까지 집에 돌아올 거에요.)
A: Why are you so late? (너는 왜 이렇게 늦었어?) B: I missed the bus. (저는 버스를 놓쳤어요.)
A: Who broke the TV? (누가 TV를 고장냈어?) B: Tom did it. (톰이 그랬어요.) C: No, James did it. (아니야, 제임스가 그랬어요.)
Now, 7 translations.
(I) didn't say he stole the money. (Someone else said it.) 그가 그 돈을 훔쳤다고 '내가' 말하지 않았어. (다른 어떤 사람이 말했어.)
I (didn't) say he stole the money. (That's not true at all.) 나는 그가 그 돈을 훔쳤다고 말하지 '않았어'. (그건 사실이 아니야.)
I didn't (say) he stole the money. (I only suggested the possibility.) 나는 그가 그 돈을 훔쳤다고 '말하진' 않았어 . (나는 단지 가능성을 제시했을 뿐이야.)
I didn't say (he) stole the money. (I think someone else took it.) 나는 '그가 or 걔가' 돈을 훔쳤다고 말하지 않았어. (내 생각엔 다른 사람이 그랬다고 생각해.)
I didn't say he (stole) the money. (Maybe he just borrowed it.) 나는 그가 그 돈을 '훔쳤다'고 말하지 않았어 . (아마도 그냥 빌렸을 거야.)
I didn't say he stole (the) money. (but rather some other money.) 나는 그가 '그' 돈을 훔쳤다고 말하지 않았어 . (다른 돈을 훔쳤지.)
I didn't say he stole the (money). (He may have taken some jewelry.) 나는 그가 '돈'을 훔쳤다고 말하지 않았어 . (아마도 그는 어떤 보석을 훔쳤을 거야.)
In Korean you say the word in single quotation marks with a stress in order to change your meaning just like in English."
Do you think this distinction is completely correct?