Greek is a strongly inflected language, which means that words are given a different ending depending on how they are used in a sentence.  These differing word forms denote cases, as follows for the Greek: Theos (Strong's G2316, God):

Theon = Accusative case: the object receiving the action of a verb.

Theos = Nominative case: a noun or pronoun which is the subject of a sentence or verb.

Theou = Genetive case: a noun that is the subject of an adjective or adverb to show ownership

TheO = Dative case: denotes the relationship of an indirect object to a verb - it is used to denote the person or thing interested or affected by something.

The above forms of Theos all mean ‘God’.  The change of ending is grammatical only.

There is also the Vocative case, which is used when addressing (speaking directly to) someone.