Masculine nouns in -ος with penult or end stress keep that stress throughout:σκύλος, σκύλο, σκύλουαριθμός, αριθμό, αριθμούMany nouns in -ος with antepenult stress shift the stress to the penult in the gen sg., gen. pl., and acc. pl.:άνθρωπος, άνθρωπο, άνθρωπε, άνθρωποι, -but- ανθρώπου, ανθρώπων, ανθρώπουςκύριος, κύριο, κύριε, κύριοι, -but- κυρίου, κυρίων, κυρίουςThese nouns are usually survivors of ancient (Classical) Greek where this type of stress shift was mandatory for all antepenult-stressed nouns and adjectives with a long ultima (final syllable), which the gen. sg., gen. pl. and acc. pl. had.However, unlike in Classical Greek, most modern adjectives keep the same stress throughout, including masculine adjectives in -ος with antepenult stress:(sg.) όμορφος, όμορφο, όμορφου, όμορφε, (pl.) όμορφοι, όμορφους, όμορφων(sg.) κόκκινος, κόκκινο, κόκκινου, κόκκινε, (pl.) κόκκινοι, κόκκινους, κόκκινων
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