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Your Korean looks nearly perfect.
I have not eaten anything since breakfast. (past or present is better?)
=> "아침 식사 이후 아무 것도 먹지 않았어요." is much better and natural.
But, be careful to use present tense sometimes.
아침 식사 이후 아무 것도 먹지 않아요.
=> I usually don't eat anything after breakfast.
I have not eaten anything, I mean, since breakfast.
아무 것도 먹지 않았어요, 아니(oops, I mean), 아침 식사 이후요.
=>아무 것도 먹지 않았어요, 사실은(in fact/ 아, oops, I mean), 아침 식사 이후요.
'아니' means actually 'no' and is quite informal expression.
So if someone ask you a question and there is some wrong information,
you want to correct it, then you can say
아니(요), 사실은~
'아,' is a kind of exclamation and you can use it in a sentence to add some additional information as well. Also, '사실은~' means 'in fact' and we can use the expression when we want to explain
something unknown to the listener(s).
모하비씨가 적은 내용이 거의 정확해요. 사실, '아니'에 대한 이해만 조금 다를 뿐,
아주 정확한 한국어 사용자처럼 보이세요.
Good luck!
