The preposition за changes the meaning of the verb бояться.
боя́ться – to be afraid of something or somebody
The verb takes the genitive case to indicate what or who someone is afraid of.
Моя́ ко́шка бои́тся воды́. My cat is afraid of water.
Студе́нты боя́тся экза́менов. Students are afraid of exams.
Бизнесме́ны боя́тся кри́зиса. Businessmen are afraid of crises.
Sometimes we use the infinitive after the verb.
Я бою́сь опозда́ть на рабо́ту. I am afraid of being late for work.
Студе́нты не боя́тся де́лать оши́бки. Students are not afraid of making errors.
Ты бои́шься води́ть маши́ну. You are afraid of driving a car.
боя́ться за – to worry about something or somebody
The verb takes the accusative case after the preposition за to indicate a person or thing someone is worrying about.
Я бою́сь за тебя́. I am worrying about you.
Я бою́сь за твоё здоро́вье. I am worrying about your health.
