The concept
English needs two words — did + know — to ask about the past. Spanish does it with one inflected verb. But "know" is special: Spanish has two verbs for it, and which one you use changes depending on whether the knowing was an ongoing state or a sudden discovery.
The two key translations
"Did you know?" — ongoing background knowledge (imperfecto)
"Did you find out?" — moment of discovery (pretérito)
When to use which
A simple test: ask yourself — was the knowing already there, or did it just happen?
"Did you know he was married?" → ¿Sabías que estaba casado? — you are asking if they had this knowledge in the background.
"Did you know (find out) that he got fired?" → ¿Supiste que lo despidieron? — you are asking about the moment they learned the news.
Examples
Imperfecto — ongoing knowledge
Did you know tomorrow is a public holiday? (imperfecto de indicativo)
Did you know your boss speaks five languages? (imperfecto de indicativo)
I didn't know you lived so far from work. (imperfecto de indicativo)
Did you know that restaurant has been closed for a month? (imperfecto de indicativo)
We didn't know the flight left so late. (imperfecto de indicativo)
Pretérito — moment of discovery
When did you find out you had been accepted? (pretérito)
He found out through a friend, not through his family. (pretérito)
Did you find out anything about the exam result? (pretérito)
We didn't find out the truth until much later. (pretérito)
How did you find out where she lived? (pretérito)
Common mistakes
✗ ¿Hiciste saber que venías?
✓ ¿Sabías que venías?
Never translate "did" directly — Spanish doesn't need it.
✗ ¿Sabiste que mañana es festivo?
✓ ¿Sabías que mañana es festivo?
Background knowledge → imperfecto, not pretérito.
Quick reference
Imperfecto
Pretérito
Imperfecto
Pretérito