poraditi

1. (na + loc.) to work on; to improve
   Moraš malo da poradiš na svom engleskom. — You need to work on your English a bit.
   Trener kaže da treba da poradimo na odbrani. — The coach says we need to work on our defence.
2. (na tome da) to make an effort toward something; to work on making something happen
   Poradiću na tome da stignemo na vreme. — I'll work on making sure we arrive on time.
3. to do some work; to work for a while
   Poradio je malo po kući. — He did a bit of work around the house.

In the sense 'to work on something' almost always used with na + locative.