To understand this, you've gotta be able to understand the difference between the particle -는 (neun) and the particle -가 (ga)
-는 (neun) places the emphasis on the verb- what is being done.
-가 (ga) places the emphasis on the pronoun- who is doing it.
저는 (Choneun) is the polite version of 나는, which means "I" (with emphasis on the verb)
Ojae mwo haesseo?
What did you do yesterday?
If the person is around the same age as you, or younger you can say:
나는 서울로 갔어
Naneun Seoul-lo gasseo.
I went to Seoul. (The emphasis of this sentence is on the fact that you went to Seoul)
If the person is older than you, or more senior, you can say:
저는 서울로 갔어요
Choneun Seoul-lo gasseoyo
This still means "I went to Seoul" but it's more polite.
Like I said before, -가 places the emphasis on who is doing the verb.
재가 (chega) is the polite version of 내가 (naega)
여기서 호주사람 있지?
Yeogi seo hoju saram it ji?
There's an Aussie here, right?
You can answer:
응, 내가 호주사람이야. (informal)
Eung, naega hoju saram i ya.
Yeah, it's me that's Australian.
네, 재가 호주사람 입니다. (Polite)
Neh, Chega hoju saram imnida.
Hope this helps!