The third declension, where most nouns live

The third declension is the biggest family of Latin nouns and holds many of the commonest, among them rex, corpus and urbs. Its genitive singular ends in -is, and that genitive hands you the stem the nominative hides. Here are its three patterns side by side.

Read across: the endings barely change. The neuter has no separate accusative and a plural in -a; the i-stem differs in one slot, -ium. Everything hard about this declension is in the first row.

The genitive is not optional here

In the first two declensions the nominative usually shows you the stem. Here it often does not, so the genitive is the only way into the noun:

Every other case is built on that stem, never on the nominative:

• Regem video. - I see the king.
• Hominem video. - I see the person.
• Militem video. - I see the soldier.

Paradigm one: consonant stems

This is the default pattern. Rex, regis, king, masculine:

Seven endings to learn, and -es and -ibus each fill two slots. A feminine noun takes exactly the same endings: lex, legis, law, gives legem, lege, legum.

• Nomen regis scio. - I know the king's name.
• Legem non habemus. - We do not have a law.
• Milites veniunt. - The soldiers are coming.

Paradigm two: neuters

Every neuter noun in Latin, in every declension, has one form for subject and object, and in the plural that form ends in -a. Corpus, corporis, body:

Tempus, genus, opus and caput all run this way:

• Corpora in terra sunt. - The bodies are on the ground.
• Nomina eorum scio. - I know their names.
• Tempus fugit. - Time flies.

The cost: on a neuter the ending cannot tell subject from object, so the verb and the other nouns settle it.