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In Korean, subject marking particles 이 (i) and 가 (ga) are used to mark the subject of a sentence. The choice between them depends on whether the preceding noun ends in a consonant or a vowel.
이 (i) is used after a noun that ends in a consonant. And 가 (ga) is used after a noun that ends in a vowel.
Examples:
If the noun ends in a consonant:
책 (book) → 책이 (chaek-i)
If the noun ends in a vowel:
학교 (school) → 학교가 (hak-gyo-ga)
