Whose: the genitive

The genitive ties a noun to another noun: of, or English 's. The dative ties a person to the verb: to or for, the one the action reaches. Between them they do the work English spreads across of, 's, to, for and a shelf of verbs.

The first two are genitive, the last three dative. A quick test: delete the verb. Liber regis still holds together; regi is left waiting for something to happen to the king.

The forms, in one place

Singular:

Plural:

Two things cause most of the trouble. The dative plural is always identical to the ablative plural, in every declension. And terrae is genitive singular, dative singular and nominative plural at once: one form, three jobs.

The genitive glues noun to noun

A genitive belongs to another noun, usually right beside it, before or after. Find that noun first.

Possession and belonging:

• filius Dei - the son of God
• Agnus Dei, qui tollis peccata mundi - Lamb of God, who take away the sins of the world (the Mass)

The whole that a part comes from (the partitive):

• pars militum - part of the soldiers
• nemo nostrum - none of us

Latin takes this further than English. Neuter words of quantity take a genitive where English uses a plain adjective or nothing at all:

Description, a genitive with an adjective, says what sort of person someone is:

• vir magnae virtutis - a man of great courage

The adjective carries the description, so it cannot be dropped: vir virtutis on its own is not idiomatic.

Subjective or objective: amor Dei can be the love God has or the love directed at God. Latin does not choose for you:

• timor hostium - the enemy's fear, or fear of the enemy

Christian writers sometimes play on both readings. Read both, and take the one the passage supports.