If the captain had been more careful,he could have prevented

对过去发生的事情作出与事实相反的推论,条件句用had + -ed,主句用should/ could /would have + -ed.
should/ could /would have + -ed可以表示“本来可以.,但是不是那样发生的,与事实相反;或者：“应该/可能已经...”,表示推测,不一定与事实不符.
所谓动词原形或者不定式,其实也有多种形式：do (不能用did,does）； have done （不能用had,has）；be done （被动）； be doing进行（当然不能用am ,is are,was ,were); have been doing (完成进行)