Latin · Chapter 3 · Lesson 8 · Grammar primer
The third declension, where most nouns live
Rex, regis. Mors, mortis. Miles, militis. The declension with the most members and the least predictable nominative - which is why the genitive is the form worth memorising.
The short version
The nominative hides the stem, so learn the genitive
The third declension is the biggest family in Latin and holds about a third of the nouns in this course. Nothing in its nominative tells you what the other cases are built on: corpus becomes corpor-, nomen becomes nomin-, iter becomes itiner-, homo becomes homin-, rex becomes reg-. The genitive singular is the form that hands you the stem, which is why every dictionary prints it and why a noun learned without it is one you can recognise and cannot use.
iter, itineris
I-stems differ in one slot, and it matters
Consonant stems take -um in the genitive plural and i-stems take -ium: regum, but urbium, navium, partium. You spot an i-stem by a nominative in -is or -es with the same number of syllables as the genitive, or by a monosyllable ending in two consonants. Neuter i-stems in -e go further, with -i in the ablative singular and -ia in the plural. Gender, though, is not predictable from the ending at all: rex is masculine, urbs feminine, corpus neuter.
urbs, urbis, urbium
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