"for breakfast" "in the morning" what's the difference?
It's the postposition. For "in the morning" you would utilize "에". "으로" has several uses including showing direction, or through what means something is accomplished or, as in this situation, for what or to what purpose.
The word '아침' may be used as an abbreviated form of '아침 식사.'
아침 식사(를) 하다
These are practically the same.
What do you eat for breakfast usually? - not accepted, why?
weird, 普通 In JAP and CHN doesn't mean usually
doesn't in Chinese, but in Japanese you can use either 普段・普通 to mean usually