The subject is a noun (or pronoun) which does the action expressed by a verb (if the verb is in the active voice) or has the action done to it (if the verb is in the passive voice).
In English, the subject
must always be expressed (by a noun, pronoun, or something functioning as
a noun). In Greek, since person is expressed in the ending of a verb, the
subject need not be expressed if the subject is a pronoun. In fact, if the
subject is a pronoun, rarely will Greek use more than the verb itself to express
both the subject and the predicate.