Here are samples for helping your understanding.
I have this 나는 이것을 가지고 있다. (Objective)
This is mine. 이것은/이것이 내 것이다. (Subject)
Noun + 을/를 is used as a direct objective in the sentence, otherwise Noun+이/가 leads a sentence as a subjective. But sometimes Noun+이/가 has a role of a indirect objective in the middle of the sentence when it especially indicates a person.