kucati

1. to knock
   Neko kuca na vrata. — Someone is knocking at the door.
   Uvek kucaj pre nego što uđeš. — Always knock before you come in.
2. (on a keyboard) to type
   On veoma brzo kuca na računaru. — He types really fast on the computer.
   Kucam ti poruku, sačekaj malo. — I'm typing you a message, hang on.
3. (of a heart or clock) to beat; to tick
   Srce mi je kucalo kao ludo. — My heart was pounding like crazy.
   Čuje se kako sat kuca na zidu. — You can hear the clock ticking on the wall.
4. (nails, pegs) to hammer in; to drive in
   Ceo dan je kucao eksere u dasku. — He spent the whole day hammering nails into the board.
   Nemoj da kucaš eksere u zid bez dozvole. — Don't hammer nails into the wall without permission.

You knock na vrata (+ acc.); a single knock is kucnuti.