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).
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.