This is the 469th most frequent Serbian word.
учествују
"Учeствују" means "they participate" or "they take part."
Here, 'учествују' is used as the present tense of the verb meaning 'participate,' indicating an ongoing action of involvement.
Студенти учествују у пројекту за заштиту животне средине.
The students are participating in the environmental protection project.
In this sentence, 'учествују' is in the interrogative form, asking about the frequency of participation in the activities.
Да ли они често учествују у оваквим активностима?
Do they often participate in such activities?
Here, the use of 'учествовали' (past tense form) indicates completion of the participatory action despite the circumstance.