Noun + 밖에 negative ending : This pattern basically means 'only.' This pattern requires a negative tense after 밖에 such as 없다 or any other negation (안 ~, 못 ~, etc). 밖에 means outside, so saying N밖에 없다 would mean there is nothing besides N, you don't have anything outside of N, or you only have N. For example: '1000원밖에 없어요' means I only have 1000원, I don't anything besides 1000원 and literally means I don't have anything outside of 1000원. e.g. 한국말을 조금밖에 못해요. I only speak a little Korean. 어제 두 시간밖에 못잤어요. I slept only 2 hours last night. 이가 아파서 물밖에 못 먹어요. I have a toothache, so I can eat only water. A : 한국말 하세요? A : Do you speak Korean? B : 아니요. 영어 밖에 못 해요. B : No, I can only speak English. 지금 200원밖에 없어. I have only 200 won now. 저는 술을 한 잔밖에 못 마셔요. I can drink only a cup of alcohol. A : 한국인 친구 있어요? A : Do you have Korean friends? B : 한 명 밖에 없어요. B : I have only one. 10분 밖에 안 남았어. We have only 10 minutes. A : 펜 하나 더 있어? A : Do you have a spare pen? B : 미안해. 하나밖에 없어. B : Sorry, I have only this one. A : 동전있어? A : Do you have a coin? B : 지폐밖에 없어. B : Sorry, I have only paper money.

모른다 means ”don’t know” and “A + 밖에” means “except A” so It means that only brother and I know and another one doesn’t know.