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What is the difference between 예전에 and 옛날!?

Simply put, 예전에 is used when describing something that happened sometime ago and 옛날 is for a very long time ago (ie back in the days...).

The difference is the time particle '에' in the noun "예전".
It makes the noun "예전" like an adverb.

e.g.
예전 = 옛날 (both are nouns)
예전에 = 옛날에 (will be followed by "something happened")

아침 = the morning
아침에 = in the morning


\^o^/