Verb → subject → object
In شَرِبَ الْوَلَدُ الْمَاءَ:
● شَرِبَ is the verb.
● الْوَلَدُ is the nominative subject.
● الْمَاءَ is the accusative object.
Ask 2 questions: Who performed the action? That is the subject. What received the action? That is the object.