There is a subgroup of feminine nouns ending in –ος , exactly like masculine nouns.

There are only a few feminine nouns ending in –ος

  η μέθοδος [method], η οδός [ street], η είσοδος [ entrance] η έξοδος [exit],   η παράγραφος [paragraph], η παράμετρος  [parameter], η ψήφος [vote]

These feminine nouns make the plural  in –οι exactly like masculine nouns

examples

η μέθοδος– οι μέθοδοι [ method- methods]

η οδός–   οι οδοί [street-streets]

You can also read: Neutral nouns in -ος

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