은/는 sets the topic, and often marks the subject as well just like 이/가. It puts the noun in the foreground. 이/가 marks something as the subject, but not necessarily as the topic. It quietly supplies the necessary information. 을/를 marks something as the object of a verb. It is often omitted, but it's good to think of it as always necessary. - 나는 너를 사랑한다. Sets 나 as the topic, to talk about it as the main topic. 너 is the object for 사랑한다.. - 나는 네가 한 말을 기억해. 나 is the main topic/subject, 너(네) the subject of the sub-phrase 네가 한, 을 the object. - 우리가 처음 만난 것은 작년이 아니라 재작년이었어. Main subject has 은, subjects of supporting phrases have 가, 이. - 1번을 누르지 말고 2번을 눌러. Don't press the #1 button. Press #2 button. - 1번은 누르지 말고 2번을 눌러. Forget about the #1 button. Just press #2. (을/를 is replaced with 은/는 for effect) (은/는 has the effect of setting the noun apart from others, like "as for (something), ...") - 모든 것은 흥할 때가 있으면 쇠할 때도 있다. Again, 은/는 for the main topic, 가 for the subject of a supporting phrase. Most grammar books and sites have a ton of information and examples on these particles.

이/가 use for e​xplaned subject. ( 은/는 use for emphasized subject ) 을/를 use for object. 내가 수지를 사랑합니다. ( 수지는 나를 모릅니다.) 선생님이 나를 칭찬했습니다.