Greek Grammar

Those ending in -eis, being more than one syllable, decline through -nt-. The monosyllabic ones decline through -n, such as heis, henos, and those derived from it. Halōs threshing floor, halōnos.
Ho bous ox. tou boos. ton boa and boun. ō bou. Ho pous foot. tou podos. ō pous. Ho pais child. tou paidos.
Those ending in -ys, which are immutable, decline through a pure -os, except for pous, podos. Ho helmins worm. tou helmirthos. Ho Tiryns. tou Tirynthos.
Every noun ending in two consonants is declined through two consonants, except for hals salt, halos.
Ho thōrax breastplate. tou thōrakos. Ho harphax rapacious/hook. tou harpagos.
Those ending in -ax decline through -k-, unless they derive from a verb ending in -zō. In that case, they decline through -g-.
Ho myrmēx ant. tou myrmēkos. Ho bouplēx ox-goad. tou bouplēgos.
Those ending in -ēx that are barytone and simple decline through -k-. Those that are compound and oxytone decline through -g-.
Ho perdix partridge. tou perdikos. Ho tettix grasshopper. tou tettigos. Those ending in -ix decline through -k-, unless the verb ends in -zō. In that case, they decline through -g-.
Ho doidyx pestle. tou doidykos. Ho onyx nail/claw. tou onychos.