popunjavati

1. (of a form) to fill in; to fill out
   Upravo popunjavam formular za vizu. — I'm filling out the visa application form right now.
   Svake godine popunjavamo istu prijavu. — Every year we fill in the same application.
2. (of space, gaps, posts) to fill
   Firma polako popunjava slobodna radna mesta. — The company is gradually filling its vacancies.
   Čitanjem popunjavam praznine u znanju. — I fill gaps in my knowledge by reading.
3. colloq. (popunjavati se) to put on weight
   Otkako je prestao da trenira, malo se popunjava. — Since he stopped training, he's been putting on a bit of weight.

The perfective is popuniti.