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못했던

couldn't, was unable to

→ 못하다
못 nail + 했 Past tense + 던 Retrospective modifier (past or ongoing past action/state)
auxiliary · past tense Rare #21,543
couldn't, was unable to Used before a noun to describe something someone could not do in the past.
KO: 어제 못했던 숙제를 오늘 했어요.
EN: I did today the homework I couldn't do yesterday.
Conjugation of 못하다
Potential past
못했어못한못했어요못했던못했죠

데이비드는 내가 무슨 생각을 하는지 알아차리지 못했던 것 이다.

David hadn't realized what I was thinking.