On the NPR news (supposedly high-class) this morning I heard, not for the first time, a reporter making an egregious mess of a word which has a Latin plural. Since there have been a number of questions and discussions of Latin recently, I thought it might be helpful to go over the rules for plurals in Latin, many of which have carried over into English, although we are in process of discarding the Latin plurals in favor of English plurals.

Latin nouns are to be found in 5 "declensions". A declension is a system for modifying, mostly by terminations, nouns to show their grammatical status or function. The declensions are classed by the nominative singular form of the nouns it contains. The 1st declension has the nouns ending in -a for all genders, such as agricola, poeta. Nouns in the 2nd declension end in -us if masculine, -a if feminine, and -um if neuter; e.g., discipulus (short 'u'), puella, bellum. 3rd declension nouns take a variety of endings. 4th declension nouns end in -es (long 'e'), such as quies; 5th declension nouns end in -us (long 'u'), such as latus.

For the record, the plural in the 1st decl. is -ae, so it's one agricola, two agricolae. In the 4th and 5th, the nominative plural is the same as the singular. As it happens, most of the nouns which have come into English are 2nd declension, with a few from the 3rd.

In the 2nd decl., the plural of -us is 'i' (long 'i'); the plural of -a is 'ae', the plural of -um is 'a' (short 'a'). So, to take examples of words brought into English, you have one alumnus (masculine) but two alumni, one alumna (feminine) but two alumnae, one stadium, two stadia, one condominium, two condominia, to be correct, although I doubt there are many people who would use stadia over stadiums. In the 3rd declension, the nom. plural is -es (long 'e'), but it isn't added to the nominative singular; in the 3rd declension, the root form of the word is found in the genitive singular, so you have to know both the nominative and genitive singular forms to know how to form the plural. The singular vir gen. viris takes the plural vires. Homo (hominis) = homines. Doctor (doctoris) = doctores; auctor = auctores.