This is the 348th most frequent Serbian word.
био
"Био" means "was" in English.
Here, 'био' is used as the past tense of the verb 'to be' ('бити') to describe a previous occupation.
Он је био учитељ.
He was a teacher.
In this sentence, 'био' asks about someone's presence at a location in the past when addressing a male.
Да ли си био у школи?
Have you been to school?
Here, 'био' is used in a subjunctive construction to express a hypothetical or imaginary scenario.