potvrđivati

1. to confirm; to acknowledge
   Hotel uvek potvrđuje rezervaciju mejlom. — The hotel always confirms the booking by email.
   Ministar niti potvrđuje niti demantuje te vesti. — The minister neither confirms nor denies the reports.
2. (of facts, data) to confirm; to bear out; to support
   Nova istraživanja potvrđuju tu teoriju. — New research supports that theory.
3. (with a nod) to nod in agreement
   Slušao me je i samo potvrđivao glavom. — He listened to me and just kept nodding.
4. (officially, in law) to approve; to ratify; to uphold
   Apelacioni sud sve češće potvrđuje prvostepene presude. — The appellate court increasingly upholds first-instance verdicts.

The perfective is potvrditi, usually used for a single completed confirmation.