When a noun as a subject ends with a last consonant(=받침), then used "이" instead of "가". e.g. 꽃(flower) =ㄲ + ㅗ + ㅊ(=last consonant, 받침) >> 꽃 + 이 not 꽃 + 가 사과(apple) > 사 + ㄱ+ㅘ(doesn't have 받침) >> 사과 + 가 not 사과 + 이 \^o^/

speaking easily, it is because of an easy pronunciation. a serial consonants is difficult to make sound a serial vowals is making a meaning vague. so, I think in many languages, one consonant(or two consonants) and a vowel appear in turns. ex 사람이~//사람(it ends with a consonant)+이(it start with a vowel-'o' is just making a position without sound. so ' ㅣ' only makes sound as a vowel) 바다가~//바다 (ends with a vowel-'ㅏ')+가(it start with a consonant-"ㄱ")