Learn Croatian: Traka je pokrenuta na vrijeme. - The belt was started on time.

The verb is pokrenuti (to start or set in motion). Regular verbs ending in -nuti take the -t ending to form the passive participle. You drop the infinitive ending -ti and add -t, giving us the base pokrenut. Because the subject traka (belt) is feminine, we add an -a to make it pokrenuta.

Why does je pokrenuta translate to "was started" when je means "is"?

In Croatian, using the present tense of the verb "to be" (like je) with a passive participle often describes a current state that resulted from a past action. Literally, it means the belt "is in a started state". In English, we express this same idea naturally by saying it "was started".

Could I use u vrijeme instead of na vrijeme for "on time"?

No, na vrijeme is the specific fixed phrase you need for "on time" or "in time". If you used the preposition u (u vrijeme), you would need to specify which time you mean, such as u to vrijeme (at that time) or u vrijeme ručka (at lunchtime).

I have seen traka used for other things. What exactly does it mean here?