Subject particle(이,가) is used only after subject.
Object particle(을,를) is used only after object.
Topic particle(은,는) can be used after subject, object, adverb.
Topic particle is mostly used for emphasizing the word.
But topic particle with subject is a little tricky.
1. 수연의 책은 여기에 있어요(O)
2. 수연의 책이 여기에 있어요(O)
3. 수연의 책을 여기에 있어요(X)
In the sentence "Soo-yeon’s book is here"
수연의 책(Soo-yeon’s book) is a subject
object particle(을,를) cannot be used with subject(수연의 책), so #3 is wrong.
Both #1 and #2 are correct. The usage is different.
#1 is the answer for "Where is Soo-yeon’s book?"
#2 is the answer for "What is here?"
A cat eats a fish.
1. 고양이가 물고기가 먹어요.(X)
2. 고양이가 물고기는 먹어요.(O)
3. 고양이가 물고기를 먹어요.(O)
Here fish(물고기) is an object.