The Korean particle -이/가 is one of the essential particles any absolute beginner needs to learn. Unfortunately, this particle with the particle -은/는 tends to cause a lot of confusion. While it is not easy to explain the difference between -이/가 and -은/는, with time you will start to see it.
The particle -이/가 is known as the subject particle because its primary function in Korean grammar is indicating the subject in a sentence. This is the part of the sentence that performs the action.
가: 책이 책상 위에 있나요?
나: 아니요, 가방 안에 있어요.
A: Is the book on the desk?
B: No, it is in the bag.
Another use for the particle -이/가 is to emphasize something. It is done when you want to emphasize the subject of a preceding sentence in the following sentence. Oftentimes this is done when answering a question.
가: 누가 아직 안 왔어요?
나: 정환 씨가 아직 안 왔어요.
A: Who hasn’t arrived yet?
B: Jeong Hwan hasn’t arrived yet.
Lastly, there is a third use. This is closely linked with both the first use and the particle -은/는. You also use the particle -이/가 to introduce new information, which is typically the subject of the sentence. However, this is generally done only once and when this ‘old’ information is then repeated, later on, you typically use the particle -은/는.
KR: 엣날에 한 할머니가 살았다. 할머니는 남편과 자식이 없었지만 검은색 고양이 두 마리가 있었다.
EN: One day an old lady lived. Grandma had no husband and children, but she had two black cats.
How To Use
Using the particle -이/가 is similar to using the particles -은/는 or -을/를. All these particles follow the same rule, the ‘batchim/no batchim’ rule. This means that you will have to use -이 when the noun ends in a batchim (consonant), but when the noun ends in a vowel (no batchim), you have to use -가. There is little else to it.
However, there are a few words you have to keep in mind because they change when you use them with the particle -이/가. The words you need to keep in mind are 누구 (who), 저 (I) and 나 (I). When you use either of those words with -이/가 they become 누가, 네가 and 내가 respectively.