Greek vs Theban

Greek and Theban both are nouns.

Word	Noun	Adjective	Verb	Adverb
Greek	Yes	Yes	No	No
Theban	Yes	Yes	No	No

As nouns, Theban is a hyponym of Greek; that is, Theban is a word with a more specific, narrower meaning than Greek:

Greek (noun)	Theban (noun)
the Hellenic branch of the Indo-European family of languages	a Greek inhabitant of ancient Thebes
a native or inhabitant of Greece	an Egyptian inhabitant of ancient Thebes

Greek (adjective)	Theban (adjective)
of or relating to or characteristic of Greece or the Greeks or the Greek language	of or relating to the Greek city of Thebes or its people or culture
	of or relating to the Egyptian city of Thebes or its people or culture