This construction means that something has/had happened or has/had not happened at a specific moment in the past. The simple past tense 었/았 indicates the completion of an action or an action in the past. If an action is completed, it is in the past tense, as in
앉았다, which means ""I sat down"" (but it may also mean ""I am in a sitting position""). The double past tense marker 었/았었 indicates either an action that was completed in the past (and then some other action came after it) or a simple experience, as in the following examples: