Grammar
-(으)시- + -었- + -어요
honor the subject in the past
Form
Verb/adjective stem + -(으)시- + -었- + -어요 — polite honorific past.
Function
Shows respect to the addressee as subject while describing a past action.
Why this form?
-았/었어요 (“polite past without subject honorific”) — Polite to the listener without a subject honorific.
Register and nuance
Subject honorification and politeness toward the listener are separate choices.
Forms not to confuse
-았/었어요
언제 알았어요? — When did you find out?
Examples
어-어떻게 아셨어요?! — H-how did you know?!
이 소식은 언제 아셨어요? — When did you hear the news?