았/었어서
I know 아/어서 can't be in past tense but I still see koreans use 았/었어서. what does it mean when its like that [았/었어서]

Question

Answer

As you know 았/었어서 is an awkward expression.

When someone uses it, he/she might want to emphasize that the first action was completed before the second action.
But, it still sounds awkward to most people.
 
그때 휴대폰 수리를 맡겼어서 전화를 못 받았어 - awkward
그때 휴대폰 수리를 맡겨서 전화를 못 받았어 - natural

Answer

Dang I thought I knew the answer to this but apparently I dont. Interested in an answer though!