The genitive case is the Latin grammatical case of possession that marks a noun as being the possessor of another noun, for example in English "Popillia's book" or in "board of directors", but it can also indicate various relationships other than possessions.

The genitive is the only grammatical case in Latin which is different in each of the five declensions, so the genitive case serves to distinguish the single declensions from each other.

Here are the basic and very general rules for making a genitive in singular:

  • If a word ends in "-us" or "-um", then the genitive ends in "-i". Tullius becomes Tullii; forum will be fori.
  • If a word ends in "-a", then the genitive ends in "-ae". Livia becomes Liviae.
  • If a word ends in "-o", then the genitive ends in "-onis". Cicero becomes Ciceronis.
  • Many other words change their ending to "-is" whose rules are more difficult and are not detailed here. Here are some just for example:
Audens in genitive becomes Audentis,
Venus in genitive is Veneris,
homo in genitive is hominis,
consul in genitive is consulis
praetor in genitive is praetoris and so on.
  • Some common nouns ending in "-us" change their ending to long "-ús" in genitive, for example:
Senatus in genitive is Senatús,
exercitus in genitive is exercitús and so on.