When Latin nouns are listed for memorisation they are listed with the nom. sg., the gen. sg. and their gender. E.g. agricola, agricolae, masculine. Why are each of these forms necessary for memorisation? Can they not be worked out from each other?
Can they not be worked out from each other?
No! That's the whole point for learning all of them.
Why are each of these forms necessary for memorisation?
That is the minimal amount of information to deduce how to use the noun. Drop any one of the three, and you will have some ambiguities.
To be able to fully use a noun, you need to know all its forms and its gender. You cannot reliably deduce the gender from the forms, although one can often guess well and there are some clear cases. In fact, the singular nominative is the least necessary piece of information out of the three, but it is sometimes necessary and in my opinion always useful.
Here are some examples:
The words agricola and traha are declined exactly the same way: agricolam/traham, agricolae/trahae and so on. But they are different gender, and there is no way you can tell that by looking at the forms.
There is a way to guess these right: A sledge (traha) does not have a natural gender, so it is feminine like most words of the first declension. A farmer is primarily male, so agricola is masculine instead. But sometimes you would guess wrong, and you should not rely on guesswork. That's why you should learn the gender and why a dictionary will give it to you.
The words agricola, problema, and castra all seem to end in -a. But based on that alone you can't figure out the other forms. With the genitives — agricolae, problematis, castrorum — you know much more. The declensions are first, third, and second, and castra is plural. (The singular castrum exists also but is rarer.)
If you have learned the case endings well, you will recognize that castra has to be neuter, and it is indeed. As discussed earlier, agricola could be feminine or masculine based on form. But with a third declension word like problema you cannot tell the gender by the nominative and the genitive. And you cannot tell the singular accusative and plural nominative and accusative without knowing the gender. Therefore a dictionary will tell you that it's neuter.
It helps if you remember that all words in -ma, gen. -matis, are neuter (and Greek loans). But you don't have to know such things. Just memorize the gender with this word or look it up in a dictionary.
To give a single fuller example, you need to know both the genitive and the gender to figure out the singular accusative of problema. It is not problemam or problematem but problema.
Joonas's answer is spot-on, but to give some more illustrations:
Quite a lot of Latin nouns end in -us; it's one of the best-known features of the language. But they don't all decline the same!
- Servus "slave", genitive servī
- Tempus "time", genitive temporis
- Scelus "crime", genitive sceleris
- Manus "hand", genitive manūs
You need the genitive to know which of these patterns it follows.
You also need the gender to deal with adjectives, because (most) adjectives reflect the gender of the noun they're modifying, not the declension pattern. These three nouns all follow the -us, -ī pattern, but have different genders:
- Servus bonus "a good slave", masculine
- Quercus bona "a good oak", feminine
- Vulgus bonum "a good crowd", neuter
All three of these decline like servus (with a genitive in -ī), but their genders are different.
And unfortunately you can't always derive the nominative from the genitive, either:
- Hominis "of the person", nominative homō
- Nōminis "of the name", nominative nōmen
- Turris "of the tower", nominative turris
- Tūris "of the incense", nominative tūs
- Honōris "of honor", nominative honor
- Floris "of the flower", nominative flos
- Corporis "of the body", nominative corpus
So in the end, you need all three of the nominative, genitive, and gender; no one or two of those will suffice.
Joonas covers the reasons for learning the three pieces of information: the nominative, the genitive and the gender. Draconis provides a thorough selection of examples of why you need both forms to properly decline nouns and adjectives. I will in my answer try to list some general rules for how to determine the gender of a noun without the information provided by the dictionary, and from this at the end list some false friends which demonstrate why learning both forms and gender are necessary.
Grammatical gender characteristics of the five declensions
Characteristic: Nominative in ‑a, genitive in ‑ae.
Genders: First declension nouns are as a rule feminine, but words denoting men are masculine: nauta (sailor), agricola (farmer).
Notes: Although genitive singular is by the time of Classical Latin ‑ae, words such as paterfamiliās < pater familiās demonstrate the older genitive singular ‑ās. According to Allan A. Lund (I seng med romerne), māter familiās did not exist as a legal term the same way paterfamiliās did.
Further note that Greek words in ‑ās, ‑ēs and ‑ē are generally declined as regular first declension nouns in the plural, but in the singular use a mix of Latin and Greek declension rules:
|Aenē‑||‑ās||‑ā||‑ān or ‑am||‑ae||‑ae||*‑ā|
|Pers‑||‑ēs or ‑a||‑ēs or ‑a||‑ēn or ‑am||‑ae||‑ae||‑ē or ‑ā|
Characteristic: Nominative in ‑us, ‑er or ‑um (neuter), genitive in ‑ī.
Genders: Masculine (‑us or ‑er) or neuter (‑um). Names of cities, islands, countries and trees are female. The words vīrus and vulgus are neuter.
Notes: Nouns ending in ‑ius and ‑ium get a genitive of ‑iī or ‑ī. Nouns ending in ‑ius get a vocative of ‑ī: cārissime fīlī. Genitive plural ‑ōrum often becomes simply ‑ūm for coins and measurements.
Characteristics: Multiple nominative endings; genitive singular ‑is.
Genders: All three.
- Masculine identifier:
- Feminine gender: arbor, arboris
- Neuter gender: iter, inineris; aequor, aequoris; marmor, marmoris. Note that these have short ‑or, as per 3.5 above.
- Feminine identifier:
- Masculine gender: ōrdō, ōrdinis; dens (or dēns), dentis; fons (or fōns), fontis; mons (sim.), montis; pons (sim.), pontis; grex, gregis; mensis (sim.), mensis; ignis, ignis; fīnis, fīnis.
- Feminine or masculine gender: cīvis, cīvis.
- Neuter identifier:
|Gender||Category||Stem ending||Nominative ending|
|m/f||consonant stem||g, c, b, p, d, t||added ‑s|
|m/f||consonant stem||l, r||none|
|m/f||consonant stem||‑on||‑n dropped|
Characteristic: Nominative in ‑us, genitive in ‑ūs.
Genders: Words in ‑us are masculine; words in ‑u are neuter.
- domus, ‑ūs is feminine and optionally follows second declension in accusative and genitive plural.
- senātus, ‑ūs can have genitive singular per second declension.
- These words are also feminine:
- manus, ‑ūs
- porticus, ‑ūs
- tribus, ‑ūs
- īdus, ‑uum
- All names for trees.
Characteristic: Nominative in ēs, genitive in ēī (stem ends in vowel) or eī (stem ends in consonant).
Genders: Mainly feminine.
Exceptions: The word diēs is unusual, in usually being masculine in the singular and always in the plural. If feminine singular, it means ‘deadline’, indicates some specific dates, references the passing of time, or references the deity. To make it more complicated, merīdiēs is always masculine.
|Genera||NOM sg||VOK sg||AKK sg||GEN sg||DAT sg||ABL sg||NOM pl||VOK pl||AKK pl||GEN pl||DAT pl||ABL pl|
|m, n (f)||us/um||e/um||um||ī||ō||ō||ī/a||ī/a||ōs/a||ōrum||īs||īs|
|m, f, n||-/is/e||-/is/e||em/im||is||ī||e/ī||ēs/a||ēs/a||[ēs/īs]/a||ium||ibus||ibus|
|m, n (f)||us/ū||us/ū||um||ūs||uī/ū||ū||ūs/ua||ūs/ua||ūs/ua||uum||ibus||ibus|
|m, n (f)||ēs||ēs||em||ēī/eī||ēī/eī||ē||ēs||ēs||ēs||ērum||ēbus||ēbus|
- 1st declension
- NOM sg. ‑a can be mistaken for NOM/VOK/AKK n. pl. of both 2nd and 3rd declension; knowing the genitive removes ambiguity.
- Some words are masculine; knowing the gender removes ambiguity.
- 2nd declension
- NOM sg. ‑us can be mistaken for NOM/VOK singular of 4th declension; knowing the genitive removes ambiguity.
- NOM sg. ‑um can be mistaken for 4th AKK sg or the short form ‑um of 2nd GEN pl; knowing the genitive removes the ambiguity.
- 3rd declension
- NOM f arbor, arboris, NOM n iter, itineris, NOM n aequor, aequoris, NOM n marmor, marmoris can be mistaken for masculine; knowing that they all have short ‑oris and not ‑ōris and of course the genders of the words, removes ambiguity.
- NOM f sg. ‑tās can be mistaken for AKK f pl. ‑ās; knowing the genitive removes ambiguity.
- NOM f sg. ‑tūs can be mistaken for ‑ūs of both 2nd (‑us, if one hasn’t memorised vowel quantity) and 4th; knowing the genitive and gender removes ambiguity.
- NOM f sg. ‑tō can be mistaken for 2nd DAT/ABL sg. ‑ō; knowing the genitive and gender removes ambiguity.
- NOM f sg. ‑is can be mistaken for 1st/2nd DAT/ABL pl. ‑īs; knowing the vowel quantity, the genitive and the gender removes ambiguity.
- NOM n sg. ‑us can be mistaken for 2nd m sg. or 4th m/f sg.; knowing the genitive and gender removes ambiguity.
- 4th declension
- NOM m sg ‑us can be expected to have genitive of ‑ī (2nd) or ‑is (3rd); knowing the genitive removes ambiguity.
- NOM f sg domus can be expected to be masculine; knowing the genitive and gender removes ambiguity.
- 5th declension
- NOM m/f sg. can be mistaken for 3rd NOM/VOK/AKK pl m/f; knowing the genitive and gender removes ambiguity.
Samson Eitrem: Latinsk grammatikk, 3rd edition by Bjørg Tosterud and Egil Kraggerud, Aschehoug, 1996. ISBN 978-82-03-32251-8