If you learn one Greek declension first, make it this one. Masculine nouns ending in -ος are the single largest and most productive noun class in the language — everyday words like ο δρόμος (the road), ο ουρανός (the sky), ο φίλος (the friend), ο κόσμος (the world) and ο άνθρωπος (the person) all belong to it. Master its endings and its accent behaviour and you can decline thousands of nouns.
The key idea for an English speaker: the noun itself changes its ending in every case. English keeps friend fixed and lets of, to, and word order do the work. Greek rebuilds the tail of the word each time. Here is the whole set.
The full paradigm
Take ο δρόμος ("the road/street"), a noun stressed on the next-to-last syllable:
| Case | Singular | Plural |
|---|---|---|
| Nominative | ο δρόμος | οι δρόμοι |
| Genitive | του δρόμου | των δρόμων |
| Accusative | τον δρόμο | τους δρόμους |
| Vocative | δρόμε | δρόμοι |
The endings themselves are the skeleton of the whole class:
| Case | Singular ending | Plural ending |
|---|---|---|
| Nominative | -ος | -οι |
| Genitive | -ου | -ων |
| Accusative | -ο | -ους |
| Vocative | -ε | -οι |
Two things jump out. First, the tell-tale -ς appears only in the nominative singular (δρόμος) and the accusative plural (δρόμους); the accusative singular drops it (τον δρόμο, not ×τον δρόμος). Second, the nominative and vocative are identical in the plural (οι δρόμοι / δρόμοι). These patterns repeat across every masculine class.
Ο δρόμος για το χωριό είναι κλειστός λόγω χιονιού.
o dhrómos ya to chorió íne klistós lógho chioniú
The road to the village is closed because of snow.
Στην άκρη του δρόμου υπάρχει ένα περίπτερο.
stin ákri tu dhrómu ipárchi éna períptero
At the end of the street there's a kiosk.
Πάρε τον δρόμο αριστερά μετά τα φανάρια.
páre ton dhrómo aristerá metá ta fanária
Take the road on the left after the traffic lights.
Where is the stress? Three accent types
The endings above never change — but where the stress sits depends on the individual noun, and it determines whether the word shifts its accent in the genitive. Greek masculine -ος nouns come in three accentual flavours:
- Oxytone — stress on the last syllable: ο ουρανός, ο γιατρός (doctor), ο θεός (god).
- Paroxytone — stress on the next-to-last syllable: ο δρόμος, ο φίλος, ο κόσμος.
- Proparoxytone — stress on the third-from-last syllable: ο άνθρωπος, ο δάσκαλος (teacher), ο πόλεμος (war).
Only the proparoxytone group shifts its accent, and only in specific cases. The oxytone and paroxytone groups keep the stress exactly where it is throughout the paradigm. Compare ουρανός (oxytone — accent glued to the ending) with the shifting άνθρωπος below.
Ο ουρανός απόψε είναι γεμάτος αστέρια.
o uranós apópse íne yemátos astéria
The sky tonight is full of stars.
Το χρώμα του ουρανού άλλαξε λίγο πριν τη βροχή.
to chróma tu uranú álakse lígho prin ti vrochí
The colour of the sky changed just before the rain.
The accent shift in proparoxytone nouns
One thing about this class has to be flagged, even though it is not this page's subject. The genitive endings -ου / -ων and the accusative plural -ους are historically "heavy," and they will not tolerate an accent sitting three syllables back — so a proparoxytone noun is pushed one syllable forward in exactly those three cells, and nowhere else:
άνθρωπος → του ανθρώπου, των ανθρώπων, τους ανθρώπους — but τον άνθρωπο, οι άνθρωποι, άνθρωπε. The same goes for every antepenult-stressed -ος noun: ο δάσκαλος → του δασκάλου, ο πόλεμος → του πολέμου.
Το γραφείο του δασκάλου είναι στον πρώτο όροφο.
to ghrafío tu dhaskálu íne ston próto órofo
The teacher's office is on the first floor.
Οι απόψεις των ανθρώπων διαφέρουν πάντα.
i apópsis ton anthrópon dhiaférun pánda
People's opinions always differ.
The vocative in -ε
Address a masculine -ος common noun by swapping -ος for -ε: ο φίλος → φίλε, ο κύριος → κύριε, ο γιατρός → γιατρέ. Masculine first names in -ος do the opposite and take -ο (Νίκος → Νίκο, Πέτρος → Πέτρο) — so you call a stranger κύριε but your friend Νίκο. Everything else about direct address, including the other endings, surnames and titles, is on the vocative case.
Γιατρέ, μπορώ να σας ρωτήσω κάτι ακόμα;
yatré, boró na sas rotíso káti akóma;
Doctor, can I ask you one more thing?
Νίκο, ξέχασες τα κλειδιά σου πάνω στο τραπέζι.
Níko, kséchases ta klidhiá su páno sto trapézi
Nikos, you left your keys on the table.
Everyday -ος nouns in the plural
Οι φίλοι μου έρχονται για φαγητό το Σάββατο.
i fíli mu érchonde ya fayitó to Sávato
My friends are coming over for dinner on Saturday.
Κάλεσα τους φίλους μου σε ένα ποτό.
kálesa tus fílus mu se éna potó
I invited my friends for a drink.
Common Mistakes
1. Keeping the -ς in the accusative singular. The -ς is a nominative marker; the object form drops it.
❌ Κοιτάζω τον ουρανός.
Incorrect — the accusative singular loses the -ς.
✅ Κοιτάζω τον ουρανό.
kitázo ton uranó
I'm looking at the sky.
2. Failing to shift the accent in the genitive. For proparoxytone nouns the stress must move forward.
❌ Η ομιλία του πρόεδρου κράτησε μία ώρα.
Incorrect — πρόεδρος is proparoxytone, so the genitive is προέδρου, with the accent shifted.
✅ Η ομιλία του προέδρου κράτησε μία ώρα.
i omilía tu proédhru krátise mía óra
The president's speech lasted an hour.
3. Giving the -ος class the -ες plural. The plural of this class is -οι; -ες belongs to the -ας and -ης classes.
❌ Οι δρόμες στο κέντρο είναι πολύ στενοί.
Incorrect — the -ος plural is -οι: οι δρόμοι.
✅ Οι δρόμοι στο κέντρο είναι πολύ στενοί.
i dhrómi sto kéndro íne polí stení
The streets in the centre are very narrow.
4. Using the nominative plural as an object. -οι is the subject form; the object form is -ους.
❌ Είδα τους φίλοι μου στην παραλία.
Incorrect — after τους the noun must be accusative plural: τους φίλους μου.
✅ Είδα τους φίλους μου στην παραλία.
ídha tus fílus mu stin paralía
I saw my friends at the beach.
Key Takeaways
- The -ος class is the biggest masculine group: endings -ος / -ου / -ο / -ε (sg.) and -οι / -ων / -ους / -οι (pl.).
- The -ς shows up only in the nominative singular and accusative plural; the accusative singular is bare (τον δρόμο).
- Three accent types: oxytone (ουρανός) and paroxytone (δρόμος) never move; proparoxytone (άνθρωπος) shifts forward in the genitive singular, genitive plural, and accusative plural — του ανθρώπου, των ανθρώπων, τους ανθρώπους.
- Vocative: common nouns take -ε (φίλε, κύριε, γιατρέ); masculine first names take -ο (Γιώργο, Νίκο).
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Start learning Greek →Related Topics
- Accent Shift in DeclensionB1 — Why the stress jumps forward when you decline a noun — άνθρωπος but του ανθρώπου, αυτοκίνητο but του αυτοκινήτου, κυβέρνηση but των κυβερνήσεων. The three-syllable law applied systematically, gender by gender, with tables showing exactly which cells move.
- The Vocative Case (Direct Address)A2 — How to change a noun's ending when you call or address someone: masculine φίλος → φίλε, but the name Γιώργος → Γιώργο; Γιάννης → Γιάννη; Θωμάς → Θωμά; feminine and neuter usually stay put (Μαρία!, παιδί μου!). The everyday address κύριε / κυρία and the dropped article.
- The Greek Case SystemA1 — The master map of Greek cases — nominative, genitive, accusative and vocative, what each one is for, why the ancient dative is gone and what took its place, and why the article and the adjective carry the case more visibly than the noun does.