This post describes Greek third declension nouns with consonant stems (i.e., stems ending with consonants). Third declension nouns can either be masculine, feminine or neuter in gender. Masculine and feminine second-declension nouns share case endings. Because of this, there are many that are either feminine or masculine in gender, with only the use of article to distinguish them (e.g. ὁ/ἡ αἴξ, the male/female goat).
* To decline any noun, you should take the genitive singular form (which can be found in a proper dictionary entry) and remove the ending; to that stem add the appropriate ending for the case and number required.
Unlike first declension nouns, the plural genitive of third declension nouns does not always take a circumflex on the ultima. But as with first declension nouns, when the accent of the genitive singular is on the ultima, both the genitive and dative will receive the accent, singular and plural. In the plural genitive, this accent will be a circumflex.
Nominative has no rule to determine it from the singular genitive, so it must be learned from the dictionary entry.
Masculine and Feminine Peculiarities
Most nouns of these genders take the ending -α in the accusative singular. However, when the noun stem ends in -ιδ, -ιτ or -ῑθ, and this ending is not accented, the ending is formed by dropping the final dental and replacing it with -ν.
The vocative singular is the same as the nominative singular when the ending of the nominative singular is -ξ or -ψ. This is also true when the nominative singular ends in -ν or -ρ and the ultima is accented. In all other cases, the vocative singular is the stem with any final dental dropped.
With the neuter, the accusative and vocative singular are exactly the same as the singular nominative.
Plural Dative Sound Combination
The ending -σι[ν] combines with the consonant stem causing slight changes in the resulting form.
-ψι[ν] := -π,-β,-φ + -σι[ν]
-ξι[ν] := -κ,-γ,-χ + -σι[ν]
-σι[ν] := -τ,-δ,-θ,-σ,-ν + -σι[ν]
-λσι[ν] := -λ + -σι[ν]
-ρσι[ν] := -ρ + -σι[ν]
-ᾱσι[ν] := -αντ + -σι[ν]
-εισι[ν] := -εντ + -σι[ν]
-ουσι[ν] := -οντ + -σι[ν]