이 (i) / 가 (ga)
The role of subject marking particles is much simpler than that of topic marking particles.
Nouns ending in a consonant : 이 (가방+이)
Nouns ending in a vowel : 가 (사과+가)
Bluntly speaking, the topic marking particles 은 and 는 express what the topic of the sentence is and subject marking particles 이 and 가 show what the subject of the sentence is.