ubacivati

1. (+ u + acc.) to throw in; to put in
   Ubacuje novčiće u automat za kafu. — He's putting coins into the coffee machine.
   Uvek ubacujem veš u mašinu uveče. — I always put the laundry in the machine in the evening.
2. (into text, speech) to insert; to add in
   Stalno ubacuje engleske reči kad priča. — He keeps throwing English words in when he talks.
3. colloq. (ubacivati se u razgovor) to butt in; to chime in
   Nemoj da se ubacuješ dok ja pričam! — Don't butt in while I'm talking!

The perfective is ubaciti; the destination is usually u + accusative.