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은/는 and 가/이

가/이=が

은/는=は

가가가가?

가 는 격조사 이고
는 은 보조사 입니다만..

if you have a car.. and when you say;

성능이 좋네! , this means you are satisfied with the car's performance, like" good performance!"

성능은 좋네!, this means that you are only satisfied with the cars performance so far.
like, "well, at least performance is good"

격조사 will be used to describe the main subject in the sentence and will cover more of...um..subject that is being described.
보조사 while has a limited description on the subject.

sorry for the obscureness. i hope i could explain it better. this question is hard for a normal person like me. it will be better if a teacher or a person majored in korean would explain.
well by the way great choice of languae you are learning there..i personally think korean and japanese are very similar.

example for Personal pronouns.

나가.....X (but used in local dialect) 내가 O
나이.....X
나은.....X
나는.....O




너가....O or 네가 O
너이....X
너은....X
너는....O

당신가.....X
당신이.....O
당신은....O
당신는.....X

그대가.....O
그대이.... X
그대은.....X
그대는.....O

그가......O
그이.....X (but used in local dialect)
그은.....X
그는.....O

그녀가....O
그녀이....X
그녀은....X
그녀는....O

우리가.....O
우리이.....X
우리은.....X
우리는.....O

사람가 X
사람이 O 사람이 많다.
사람은 O 그 사람은 아프다.
사람는 X

하늘가 X
하늘이 O.... 하늘이 무너지다.
하늘은 O.... 하늘은 파랗다.
하늘는 X

If you look closely those composition, you will know that noun+가/이 or noun+는/은 has a simple rules. you should look into noun's last character.

A hangul character has theree component part.

ex)
강(river), ㄱ+ㅏ+ㅇ(이응)
산(mountain) ㅅ+ㅏ+ㄴ
나, ㄴ+ㅏ+(silent)
너, ㄴ+ㅓ+(silent)

word last character
고양이(cat), 고양ㅇ+ㅣ+(silent)
인생(life), 인ㅅ+ㅐ+ㅇ(이응)
책(book), ㅊ+ㅐ+ㄱ

if last character is vowe,
이/은 X.....가/는 O

if last character is consonant(include ㅇ,이응)
이/은.....O......가/는 ........X

PS) I'm sorry, I'm not good at english.wrighting