The subject of a sentence is the thing that does the action. The VERB is the functional verb in a sentence, the one that gets the conjugation for number and tense. The object can be many things, but it is what is acted upon by the subject. In other terms, the subject is the doer, the verb is what is done and the object is what it gets done to. The reason the object is in parentheses is because sometimes verbs take no objects (The baby cries) and sometimes they take two (I gave Juan the pen).