내가 안 그랬어 = I didn't do it ⇒ It's wasn't anyone else but me who didn't do.
나는 안 그랬어 = I didn't do it ⇒ It carries out the action of not doing rather than not doing something else.
You're right, imsogood. I'm going to try to clarify it a bit more. Uh, well… My brain is still loading now.
가 / 이 are same as 는 / 은 in use but there is a fundamental difference. 가 / 이 particle puts more emphasis on the subject. Let's see one conversation.
철수 Chul-soo : 엄마, 형이 그릇 하나 깨먹었어! Mum, my older brother broke a plate!
철호 Chul-ho : 내가 안 그랬어! 철수가 그랬어! No, I didn't! Cheol-su did it!
엄마 Mom : 철수, 네가 그랬지, 아니니? Cheol-su, you did it, didn't you?
철수 Chul-soo : 나는 안 그랬어. I didn't do it.