Uses of Nouns
In a sentence, nouns can play the role of subject, direct object, indirect object, subject complement, object complement, appositive, or adjective.
Maria is happy.
Maria is the subject.
Give the books to her.
Books is a direct object; her is the indirect object.
Mary is a teacher.
Teacher is a subject complement.
I now pronounce you husband and wife.
Husband and wife are object complements.
My brother Michael is six years old.
Michael is an appositive.
He is a speed demon.
Speed is a noun which is modifying demon; speed is acting like an adjective.