has changed since
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.
It is formed by adding -았었어요 to the stems of verbs and adjectives with stems ending in the vowel ㅏ or ㅗ.
Otherwise, -었었어요 is added. For verbs ending in 하다, 했었어요 is added.
What is difference between Verb/ Adj 았/었어요 and Verb/ Adj 았/었었어요
Expresses that something simply occurred in the past or that an action or situation that ended in the past has remained that way.
Expresses a past occurrence that does not continue to the present.
Referring to a past event that is different from the present or does not continue to the present.
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