The Article and the Cases
One article, and no second one
Greek has a definite article — ὁ, ἡ, τό, our "the" — and no indefinite article at all. There is no Greek word for "a". So θεός on its own can be "God" or "a god" or simply "God" as a quality, and the translator decides from the sentence, not from a missing word. That single fact does more work in New Testament translation than any other point of Greek grammar, and it is why the article, small as it is, gets a lesson of its own. It is also the most common word in the New Testament by a wide margin: count the article across the Lantern's whole Greek text and it is roughly one word in every seven.
The article agrees with its noun
English "the" never changes. The Greek article changes to match its noun in three things at once: case (the noun's job in the sentence), number (singular or plural), and gender (masculine, feminine, neuter). So in John 1:1 the article before λόγος is ὁ, and the article before θεόν is τόν — same word, different shape, because the two nouns are doing different jobs. When you tap either one in the reader, the panel says so: "definite article · nominative singular masculine" for the first, "definite article · accusative singular masculine" for the second.
The five cases
A Greek noun's ending tells you its job, so word order is free in a way English's is not. There are five cases. Nominative: the subject — ὁ λόγος, "the Word", the thing doing the being. Accusative: the direct object, and the object of many prepositions — τὸν κόσμον at John 3:16, the world God loved. Genitive: of, from, belonging to — πατρός at John 1:14, "of the Father". Dative: to, for, with, in — ἀρχῇ at John 1:1, which is dative because ἐν takes it, and τοῖς οὐρανοῖς at Matthew 6:9, "in the heavens". Vocative: the case of being spoken to — πάτερ, "Father", the first word of the Lord's Prayer proper.
The Lord's Prayer as a grammar table
Matthew 6:9 puts four of the five cases in one line. Πάτερ ἡμῶν ὁ ἐν τοῖς οὐρανοῖς, ἁγιασθήτω τὸ ὄνομά σου. Πάτερ is vocative — someone is being addressed. ἡμῶν is genitive, "of us", which is how Greek says "our". τοῖς οὐρανοῖς is dative plural after ἐν. τὸ ὄνομα is nominative neuter, the subject of the verb that follows, and σου is genitive again, "of you". The article ὁ in the middle is vocative too, agreeing with Πάτερ across four words — the grammar holding the phrase together at a distance that English would need a relative clause to manage.
The article that is not there
Return to John 1:1: καὶ θεὸς ἦν ὁ λόγος. Both θεός and λόγος are nominative singular — the site's own parsing data says so of each — and in Greek that would ordinarily be ambiguous about which is the subject. The article settles it. λόγος has the article, θεός does not, and the articular noun is the subject: "the Word was God", not "God was the Word". Whatever more one wants to say about that clause, this much is grammar rather than interpretation, and you can now see it on the page for yourself: three words, one article, and the article is on λόγος.
Why it matters on this site
The reader gives every Greek word its parsing in words: tap ἐσκήνωσεν at John 1:14 and it reads "verb · aorist · active · indicative · third person singular". The cases are the half of that apparatus which decides what a sentence means rather than merely when it happened, and they are the tool for the questions this site actually argues — which Greek text the Lantern Direct will translate when it reaches the New Testament, and what the editions do differently where they differ.