Grammar
-(으)시- + -었- + -어요
honor the subject in the past
Form
Verb/adjective stem + -(으)시- + -었- + -어요 — polite honorific past.
Function
Combines respect for the subject with past tense; it can be used for a stranger without a large age gap.
Why this form?
-었어요 (“polite past without subject honorific”) — Marks polite past tense without honoring the subject.
Register and nuance
Subject honorification and politeness toward the listener are separate choices.
Forms not to confuse
-었어요
들었어요? — Did you hear it?
Examples
들으셨어요? — Did you hear it?
어제 잘 주무셨어요? — Did you sleep well yesterday?