Neuter Stems in -τ

118.

119. In the nominative, accusative, and vocative singular neuters take no case-ending; final -τ is dropped (40). In τὸ γάλα, γάλακτ-ος (milk) final -κτ is dropped; τέρας is another form of the stem τερατ- (46).

120. Different but related stems appear in

τὸ ὕδωρ, ὕδατ-ος water
τὸ ἧπαρ, ἥπατ-ος liver
τὸ γόνυ, γόνατ-ος knee
τὸ δόρυ, δόρατ-ος spear
τὸ οὖς, ὠτ-ός ear
τὸ φῶς, φωτ-ός light (sing. only)

a. Φῶς is contracted (hence the accent, 30.b) from older φάος, which is the common form in verse.