A Latin noun is in the fourth declension if and only if its genitive singular ends in "-us". The thematic vowel of the fourth declension is "u", which appears between the stem and all the endings except "-ibus", which is borrowed from the third declension. Fourth declension nouns are almost all masculine or neuter. The only common feminine fourth declension noun in all of Latin is "manus" ("hand") which declines just like a masculine anyway. Now on to some paradigms. We'll use "situs" ("site") as an example of a masculine fourth declension noun:

case       singular plural
nominative situs    situs
accusative situm    situs
genitive   situs    situum
dative     situi    sitibus
ablative   situ     sitibus

Note that the genitive plural has two u's in a row. Note also that the form ending in "-us" can be:

  • nominative singular
  • nominative plural
  • genitive singular
  • accusative plural
Here's a paradigm for neuters using "cornu" ("horn") as an example:

case       singular plural
nominative cornu    cornua
accusative cornu    cornua
genitive   cornus   cornuum
dative     cornu    cornibus
ablative   cornu    cornibus

The nominative and accusative plurals end in "-a", in accordance with the rules of neuters. The singular ends in "-u" in all cases but the genitive, so you have to interpret the context to figure out which case is intended.

There are no fourth declension adjectives, and there is no fourth declension in Greek.

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