It basically means very similar, I found it quite challening to explain the differnce between them, so I will give you some exmaples using those words. 그는 자기가 잘생긴줄안다 = He believs himself handsome 그는 자신이 잘생긴줄안다 = Same meaning 그녀는 자기가 잘못한걸 알았다 = she knew herself that she did wrong 그녀는 자신이 잘못한걸 알았다 = Same meaning As you can see there is not much difference. What you need to be careful with is 자신"이" and 자기"가" Was it helpful? I hope this helped you :)

자기 is like dear i think. so, for couple, they use "자기" 자신 = confidence

Hi Thomas:^) I think it is pretty difficult part. actually this word came from chinese character. they are so similar. 자기自己,자신自身 自means oneself 己means body 身means body too. exactly same. but they have some difference. 자신 and 자기 are noun, but 자기 is also a reflexive pronoun that recognized in standard Korean dictionary. For example>> "철수"는 "자기"가 가겠다고 했다. ->자기= himself= 철수 "그"는 뭐든지 "자기" 고집대로 한다. ->자기= himself= 그 "아들"은 집에 들어오자마자 "자기" 방으로 들어갔다. ->자기= himself= 아들 But recently they are usually treated samely. I hope it answered your questionX)