"그녀는 아침에 공부한다."
번역:She studies in the morning.
Generally, "at" is used with a more specific time. Of course,"at night" is an exception to that rule. However, we do also use the expression "in the night"
at the morning meal ( but in the morning)
at the evening church service (but in the evening)
at the afternoon meeting ( but in the afternoon)
Sorry, I can't think of a way to help you remember.