prevent sb from doing sth 其中sb用宾格还是主格?

prevent sb from doing sth 其中sb用宾格还是主格?
这个同学,
动词和介词后面已经要接宾语,如果代词一定要代词的宾格代词,名词 就照写,英文名词主格和宾格长相是一样的,都是同一个形式
例如：The bad weather prevented us from going out for a camp
They prevented the boy from going swimming alone in this rive