When the last letter of the last syllable of a word ends in a vowel, we use 가 (소파가)
When the last letter of the last syllable of a word ends in a consonant, we use 이 (책이)
What I am about to say is very difficult for English people to grasp. When I started learning Korean, it took me months to understand this, and it was not because it was difficult. Rather, nobody explained it to me in a way that allowed me to understand. I’ll do my best to explain it to you.
When a part of a sentence is not the main clause of a sentence, we use 이 or 가 to indicate the subject of that clause of the sentence.
These particles make it so hard for me to form sentences because I can never remember them.