бояться vs бояться за

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.

You will find more examples, practice and some idioms used with the verb БОЯТЬСЯ in the video.

https://www.youtube.com/watch?v=zBv0PywKaoY