He seemed hasty.
Is hasty a complement, perhaps a verb complement, to seemed? There is no noun in the object position. May hasty be understood as an object here, to get a complete sentence?
An object is a kind of complement—a piece of the sentence which "completes" the meaning of the verb. A "complete sentence" does not necessarily require either an object (a Direct Object or Indirect Object) or any other kind of complement; it depends on what verb is employed.
Some verbs require at least one object. We call these transitive, from Latin transire, "go across", because the action of the verb "goes across" to the object: the object "receives" the action. Some of these take both an object and a complement; appoint, for instance, and make in some of its many senses:
The president appointed [DIRECT OBJECThim] [COMPLEMENTprofessor].
Brian made [DIRECT OBJECTme] [COMPLEMENTreally angry].
With these verbs, the complement is usually a nominal which identifies the object or an adjectival which describes it.
Other verbs, which we call intransitive, do not require an object. Some of these, like be, become, seem require a complement, which is usually a nominal which identifies the subject or an adjectival which describes it.
He is angry.
He has become a prominent man.
Others, however, do not require even a complement; verbs like run and talk designate activities which are self-contained.
He is running.
He talked for several minutes.
Note that many verbs have both transitive and intransitive senses. Run, for instance, is intransitive in the sentence above; but it may also be used transitively, as when we speak of running a marathon or running a company.