kvariti

1. to spoil; to ruin; to damage
   Nemoj da mi kvariš raspoloženje. — Don't spoil my mood.
   Stalno kvari igračke koje mu kupimo. — He keeps breaking the toys we buy him.
2. (morally) to corrupt; to spoil
   Mnogi misle da društvene mreže kvare decu. — Many people think social media corrupts children.
   Baka ga previše kvari poklonima. — His grandma spoils him too much with presents.
3. (kvariti se) to go bad; to go off; to break down
   Leti se mleko brzo kvari. — Milk goes off quickly in summer.
   Ovaj auto nam se stalno kvari. — This car keeps breaking down on us.

The perfective is pokvariti; for food and machines kvariti se is mostly used.