적 as 의존 명사
Here are the formal definitions as presented in daum online dictionary:
1 용언의 관형사형 어미 ‘-을’ 뒤에 쓰여, 동작이 진행되거나 상태가 나타나 있을 때를 나타내는 말.
2 용언의 관형사형 어미 ‘-은’ 이나 명사 뒤에 쓰여, 지나간 과거의 그때를 나타내는 말.
Some common construct such as 본 적이 있다 arises from this definition of 적, yet i found it quite puzzling to see 어릴 적 instead of 어린 적. Apparently the younger years are always spoken of in the past so by definition 2 it really should be wriiten as 어린 적. What is surprising is that there is another form 어맀을 적 would someone enlighten me as to why we never write 어린 적 at all? In addition how is it different between 어릴 적 and 어맀을 적?