This is the 16509th most frequent Korean word.
사겠다던
"사겠다던" means "said they would buy" in English.
The word '사겠다던' here refers to the mention or commitment of someone ('그가') to buy something ('책') in the past.
그가 사겠다던 책을 아직도 못 샀어요.
I still haven't bought the book he said he would buy.
Here, '사겠다던' emphasizes the specific commitment or statement about making a purchase, contrasting it with the subsequent non-fulfillment of that action.
사겠다던 약속을 왜 지키지 않았나요?
Why didn't you keep the promise to buy it as you said you would?
In this case, '사겠다던' conveys Minsoo's stated intention or opinion about buying a car.