This is the 1248th most frequent Polish word.
został
"Został" means "he/she/it became" or "he/she/it was left/stayed," depending on context.
The word 'został' is used here to imply someone's transformation or becoming something (becoming a teacher in this case).
On został nauczycielem w zeszłym roku.
He became a teacher last year.
In this context, 'został' indicates that something stayed or remained in a specific place.
Mój portfel został na stole.
My wallet remained on the table.
Here, 'został' forms a part of a passive construction, indicating that an action was performed on the subject of the sentence (the building was built).