A/V-았/었었어요

has changed since

Structure
하다 contraction: 하다 → 하 + 였었어요 → 하였었어요 → 했었어요 (did back then)
Regular ㅏ/ㅗ: 잡다 → 잡 + 았었어요 → 잡았었어요 (caught back then)
Regular other vowel: 먹다 → 먹 + 었었어요 → 먹었었어요 (ate back then)
ㅏ contraction: 가다 → 가 + 았었어요 → 가았었어요 → 갔었어요 (went back then)
ㅗ + 았었어요 contraction: 보다 → 보 + 았었어요 → 보았었어요 → 봤었어요 (saw back then)
ㅐ + 었었어요 contraction: 보내다 → 보내 + 었었어요 → 보내었었어요 → 보냈었어요 (sent back then)
ㅜ + 었었어요 contraction: 배우다 → 배우 + 었었어요 → 배우었었어요 → 배웠었어요 (learned back then)
ㅣ + 었었어요 contraction: 마시다 → 마시 + 었었어요 → 마시었었어요 → 마셨었어요 (drank back then)
ㅚ + 었었어요 contraction: 되다 → 되 + 었었어요 → 되었었어요 → 됐었어요 (became back then)
Complex vowel, no contraction: 쉬다 → 쉬 + 었었어요 → 쉬었었어요 (rested back then)
ㄹ-final regular here: 살다 → 살 + 았었어요 → 살았었어요 (lived back then)
Explanation

This pattern is used to express something or a situation that occurred in the past but did not continue or something separate from the present because it occurred much earlier than when the speaker is describing it. It approximates to 'did/had (in the past)' in English.

Details

What is difference between Verb/ Adj 았/었어요 and Verb/ Adj 았/었었어요

  • Verb/ Adj 았/었어요

Expresses that something simply occurred in the past or that an action or situation that ended in the past has remained that way.

댄 씨는 작년에 한국에 왔어요.
Dane may still be in Korea. We don't know what happened after he came to Korea. He may be in some place other than Korea.
댄 씨는 서울에서 1년 살았어요.
Dane has been living in Seoul for one year. He previously lived in Seoul for one year, but we don't know where he lives now.

  • Verb/ Adj 았/었었어요

Expresses a past occurrence that does not continue to the present.

댄 씨는 작년에 한국에 왔었어요.
Dane came to Korea and later left; he is not in Korea now.
댄 씨는 서울에서 1년 동안 살았었어요.
Dane previously lived in Seoul for one year, but now he does not live in Seoul.

Kimchi Reader

Referring to a past event that is different from the present or does not continue to the present.