Difference between 이/가 and 은/는

What is 이/가 and 은/는?

이 and 가 are called subject markers and 은 and 는 are called topic markers. Both service the purpose to mark the subject of a sentence.

철수가 사과를 먹고 있어요
철수는 사과를 먹고 있어요

Both sentences mean 철수 is eating an apple. From an English perspective 철수 is the subject in both sentences.

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Conjugation rules

Conjugation rule 1:  이/은 follows nouns ending in consonants. 가/는 follows nouns ending in vowels.
Conjugation rule 2: When 가 follows 저 (me) or 너 (you), it becomes 제가/네가 not 저가/너가.
Conjugation rule 3: When 는 follows 저 (me) or 너 (you), it can become 저는/너는 or abbreviated to 전/넌.

What is the difference between 이/가 and 은/는?

The difference between 이/가 and 은/는 is not black and white. There will be many cases when either particle can be used and the meaning of the sentence will be fine. There isn’t a perfectly defined set of rules that will tell you when you should use one or the other.

There will be some hair pulling moments! However, over time you will develop a natural intuition for which one to use and when. Until then, there are some guiding principles you can fall back to.

Before learning about these guiding principles, it is important to that you understand what a “predicate” is. A predicate is the part of a clause or sentence that includes the verb and describe what the subject does. Simply put, the predicate is everything in the sentence other than the subject. 

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Principle 1: 이/가 is used when focusing on the subject. 은/는 is used when focusing on the predicate

This is the most important principle in deciding between using 이/가 and 은/는.

Consider a situation where a class of people have taken a test and are awaiting the results. One person approaches the group and says that they have seen the ranks. A conversation ensues:

A: 영희가 1등이에요! (영희 is rank 1!)
B: 그럼 누가 2등이에요? (Then, who is rank 2?)
A: 철수가 2등이에요 (철수 is rank 2)
C: 그럼 순이는 등수가 뭐예요? (Then what rank is 순이?)
A: 순이는 3등이에요 (순이 is rank 3)

Sentences 1, 2, 3 emphasis the subject (the persons ranked 1 and 2) so the subject marker (가) is used.

Sentences 4 and 5 emphasis the predicate, so the topic marker (는) is used.

Principle 2: 이/가 is used when the subject is introduced for the first time. 은/는 is used when adding more information about the subject.

Example 1:
옛날에 할아버지가 살았어요. 할아버지는 노래를 잘 했어요
Once a upon a time there was an old man. The old man sang very well.

The first sentence introduces the subject, so the focus is on the subject. The second sentence adds more information about the subject. It is clear who the subject is in the second sentence so the focus is on the predicate. 

Example 2:
어느 날 그 할아버지가 산에 갔어요. 그 산은 아주 높았어요
One day the old man went to the mountain. The mountain was very tall.

The next sentence introduces a new topic so to speak, so once again we revert back to the subject market. With the setting established for this topic, the final sentence adds new information about the mountain. The focus is on the predicate, i.e. the tall description of the mountain.

Principle 3: 은/는 is used when contrasting or comparing the subject to something else.

 Example 1:
영희는 집에 있어요. 하지만 철수는 학교에 있어요.
영희 is at home, but as for 철수 he is at school.

Example 2:
선생님: 여러분 자기 소개 해주시겠어요?
영희: 저는 영희예요
철수: 저는 철수예요

The first sentence establishes that the topic is everyone (여러분). When 영희 replies the focus has changed from the subject (여러분) to the predicate (영희예요) (principles 1 and 2).

When 철수 replies it can also be seen that the focus has changed from the subject to the predicate, or we can say the subject (철수) is being contrasted to the subject of the previous sentence (영희). I.e. you are 영희, and as for me, I am 철수. 

Principle 4: 은/는 is used when contrasting or comparing the subject to something else.

 Example 1:
영희는 집에 있어요. 하지만 철수는 학교에 있어요.
영희 is at home, but as for 철수 he is at school.

Example 2:
선생님: 여러분 자기 소개 해주시겠어요?
영희: 저는 영희예요
철수: 저는 철수예요

The first sentence establishes that the topic is everyone (여러분). When 영희 replies the focus has changed from the subject (여러분) to the predicate (영희예요) (principles 1 and 2).

When 철수 replies it can also be seen that the focus has changed from the subject to the predicate, or we can say the subject (철수) is being contrasted to the subject of the previous sentence (영희). I.e. you are 영희, and as for me, I am 철수. 

Principle 5: 은/는 is used when speaking about something in general. 이/가 is used when speaking about a specific thing.

Example 1:
사람이 누워서 자요. (This, that) person sleeps lying down.
사람은 누워서 자요. People (in general) sleep lying down.

Example 2:
사과는 빨개요. As for apples (in general), they are red.
이 사과가 파란 색이에요! This (specific) apple is blue!

Principle 6: Clauses containing a topic can’t be contained in another clause.

Example 1:
저는 (우리 아빠가 좋아하는) 과일을 사줬어요
제가 (우리 아빠가 좋아하는) 과일을 사줬어요
I bought the fruit that my dad likes. 

This sentence can’t be 제가/저는 우리 아빠는 좋아하는 과일을 사줬어요.

Example 2:
(제가 살고 있는 곳)에 영희는 살고 있어요
(제가 살고 있는 곳)에 영희가 살고 있어요
영희 lives in the place I live in.

This sentence can’t be 저는 살고 있는 곳에 영희는 살고 있어요

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Conclusion

Not all sentences will follow a set of rules. Sometimes the difference will be too minuscule to care and unless someone was to look for the difference, they would likely hear what you wanted to say and move on. Consider the following sentences:

제가 그를 사랑하기 때문에 그가 나를 좋아해요
제가 그를 사랑하기 때문에 그는 나를 좋아해요
Because I like him, he likes me.

The difference in the meaning of these sentences are so minuscule and regardless of which marker is used, the intended meaning won’t be misunderstood. Of course, there is a difference if you analyse it, but here it doesn’t really matter.