Learner's definition of PREVENT
to stop (something) from happening or existing
Seatbelts in cars often prevent serious injuries.
Can exercise and a healthy diet prevent heart disease?
The accident could have been prevented.
to make (someone or something) not do something
to stop (someone or something) from doing something
He grabbed my arm to prevent me from falling.
Bad weather prevented us from leaving.
How are you going to prevent him from finding out about the party?
— often used without from in British English