~이/~가

The selection between ~이 (i) and ~가 (ga) is purely based on the final sound of the preceding noun or pronoun. If the word ends with a consonant (batchim), you must use ~이. For instance, '책' (chaek, book) ends with 'ㄱ' (g), so it becomes '책이'. Conversely, if the word ends with a vowel, you use ~가. An example is '바나나' (banana), which ends with 'ㅏ' (a), thus becoming '바나나가'. Mastering this rule is fundamental for constructing grammatically correct and natural-sounding Korean sentences. This distinction ensures smooth phonetic transitions in speech and is one of the first grammatical points learned by Korean language students.

While both ~이/가 and ~은/는 mark nouns, their functions are distinct. ~이/가 marks the grammatical subject, often introducing new information or emphasizing the subject. For example, '친구가 왔어요' (Chinguga wasseoyo) means 'A friend came' – here, 'friend' is the subject and new information. In contrast, ~은/는 marks the topic of the sentence, which is usually already known or is being introduced as the subject of discussion. '친구는 학생이에요' (Chinguneun haksaengieyo) means 'As for the friend, he/she is a student'. Understanding this difference is crucial for conveying the intended meaning and nuance in Korean.

The particles ~이/가 are consistently used across various Korean speech levels, including formal (합니다체), polite (해요체), and informal (해체). The particles themselves do not change form based on politeness; rather, it is the verb or adjective ending that determines the speech level of the sentence. For example, '날씨가 좋습니다' (Nalssiga joeupsimnida, formal), '날씨가 좋아요' (Nalssiga joayo, polite), and '날씨가 좋아' (Nalssiga joa, informal) all use '날씨가' to mark 'weather' as the subject. This consistent usage makes ~이/가 a stable grammatical element.

~이/가 is frequently used in existential sentences with verbs like '있다' (itda, to exist/have) and '없다' (eopda, to not exist/not have). For instance, '돈이 있어요' (Doni isseoyo, I have money) or '시간이 없어요' (Sigani eopseoyo, I don't have time). It is also vital in describing states or qualities using adjectives, as in '하늘이 파래요' (Haneuri paraeyo, The sky is blue). Furthermore, when answering 'who' or 'what' questions, ~이/가 is typically attached to the answer to emphasize that it is the subject. Understanding these contexts solidifies the understanding of ~이/가's role.