posluživati

1. (guests, food) to serve
   Konobar brzo poslužuje goste na terasi. — The waiter is quickly serving the guests on the terrace.
   Domaćica je posluživala kafu i kolače. — The hostess was serving coffee and cakes.
2. (posluživati se) to help oneself
   Na švedskom stolu gosti se poslužuju sami. — At a buffet the guests help themselves.
3. formal (posluživati se + instr.) to make use of; to use
   Na ispitu se poslužuje rečnikom. — He uses a dictionary during the exam.
   Često se posluživao lažima da bi uspeo. — He often resorted to lies to get ahead.

The invitation to help oneself is usually perfective: Poslužite se!