Greek Grammar

[Words] ending in -ōn as containers original: "περιεκτικὰ" are declined with -onos: for example, koitōn bedroom, koitōnos; agōn contest, agōnos.
The Ibēr Iberian, Ibēros. The lytēr deliverer, lytēros. The aithēr ether, aitheros. The aēr air, aeros.
Those ending in -ēr that have a tau are declined with -ēros, except for astēr star, asteros; patēr father, pateros. Those without a tau are declined with -eros: for example, anēr man, aneros, which is also called andros; except for spinthēr spark, spinthēros; Ibēr, Ibēros.
The Nestōr Nestor, Nestoros. The Hektōr Hector, Hektoros.
Those ending in -ōr that are barytone not having an acute accent on the last syllable are declined with -oros: for example, Nestōr, Nestoros, except for those having a lambda or a mu, which are declined with -ōros: for example, kelōr child, kelōros; tekmōr sign/goal, tekmōros; and Roman titles: for example, diktatōr dictator, diktatōros; praitōr praetor, praitōros.

The chelidōn swallow, chelidonos, chelidoni, chelidona, O chelidōn. Dual: chelidone, chelidonoin, O chelidone.
Plural: chelidones, chelidonōn, chelidosi, chelidonas, O chelidones. The rhērōn a type of bird, possibly a heron, rhēronos.
The mēkōn poppy, mēkōnos. Of the feminine nouns ending in -ōn, those that are oxytone acute-accented on the final syllable are declined with an omicron...