> A noun in Korean can be a subject, object or predicate, and if you want to makes the noun into a subject, the subject marker 이/가 has to be added after the noun. It's all up to you how you deal with the noun with markers.
i.e. 사람(a person, noun)
사람이(subject) , 사람을(object), 사람이다(predicate, with the coupla verb 이다)
> If the noun which you want to make into a subject has a last consonant(=받침), then "가" has to be added, otherwise "이"
꽃 > 꽃이 because 꽃 has the last consonant 'ㅊ'
나무 > 나무가, 무 doesn't have the last consonant, so "가"