The Fourth and Fifth Declensions: manus, exercitus, res, dies

Latin has five declension families. You have already met the big three: the first and second and the third, which between them hold the overwhelming majority of nouns in the language. What is left is two small families that every course teaches last and every learner forgets first.

Do not forget these ones. They are small in membership and enormous in frequency. Res is rank 48 in this course, the commonest noun in Latin by a distance. Dies is 55. Domus is 95, manus 137, fides 235, usus 257, exercitus 285. Seven of the first three hundred words you learn live in the two declensions nobody bothers with.

The fourth declension: manus

Spotted by a genitive singular in -us, and confirmed by a dative in -ui and an ablative in a bare -u.

Five distinct forms across ten slots, which is thin even by Latin standards. Manus alone is four of them: nominative singular, genitive singular, nominative plural and accusative plural. That looks like a disaster and is not, because the verb tells you the number and an adjective tells you the case. Magna manus venit is a great band of men is coming; magnae manus veniunt is great bands of men are coming. The noun did not move. Everything around it did.

What is actually in it

Almost the whole declension is built off verbs, which is the pattern to notice. Take the supine stem, add -us, and you get the noun for the act of doing it:

• advenio, arrive, gives adventus, an arrival
• moveo, move, gives motus, a movement
• cado, fall, gives casus, a fall, then chance, then misfortune
• exerceo, train, gives exercitus, a trained force, an army
• peto, make for, gives impetus, an attack
• utor, use, gives usus, use and then experience

Once you see it you cannot unsee it: visus, auditus, gustus, tactus - four of the five senses are fourth-declension verbal nouns. This is where Latin puts its abstractions of action, which is why senatus, spiritus, fructus, gradus, status and metus all cluster here.

Gender is masculine by default. The exceptions worth the ink are manus and domus, both feminine. There is also a handful of neuters whose nominative ends in -u: cornu (horn, and the wing of an army) and genu (knee) are the ones you will meet. They obey the universal neuter rule that nominative equals accusative, and go cornu, cornus, cornu, cornu in the singular with cornua, cornuum, cornibus in the plural.

One footnote, offered so it does not startle you: a short list of nouns takes -ubus rather than -ibus in the dative and ablative plural, to keep clear of second-declension lookalikes. Arcubus, portubus, lacubus, tribubus. It is four or five words. Recognise it, do not memorise it.

The -us trap

This is the real difficulty with the fourth declension, and it is not the paradigm.

dominus - master, second declension, genitive domini

exercitus - army, fourth declension, genitive exercitus

Nothing in the nominative distinguishes them. The genitive distinguishes them completely, and then everything downstream diverges: domino against exercitui, dominos against exercitus. This is exactly the argument the declensions article makes for learning the citation form in full, and the fourth declension is where ignoring it costs you most.

The fifth declension: res and dies

Spotted by a genitive singular in -ei. Nearly all feminine, with dies the main exception.

The whole family is res, dies, fides, spes, acies, facies, species and a few rarities. That is it. And only res and dies are declined right through the plural: spes, acies, facies and species appear in the nominative and accusative plural and hardly anywhere else, while fides has no plural at all.

Res earns its rank by being gloriously vague. It is thing, matter, affair, situation, business, property, and it is the noun a Roman reached for when English would reach for a whole clause. Res publica, the public thing, is the state - and both halves decline, so you get rei publicae, rem publicam, de re publica. Res gestae are things done, deeds. Rerum natura is the nature of things. Res novae, new things, means revolution, which tells you a good deal about Roman politics. Qua re fused into quare, wherefore.

The dies wobble

Dies is masculine, and always masculine in the plural. It turns feminine in two cases: when it means an appointed day, a deadline (dies constituta), and when it means a stretch of time rather than a calendar day (longa dies). Gender following meaning rather than form is rare enough in Latin to be memorable.

Domus, the one that will not pick a side

Domus is fourth declension in the way that a cat is a house pet. It borrows from the second whenever it feels like it:

Then it does something no other common noun does: it keeps a locative, the ghost seventh case, and uses three bare forms for movement with no preposition at all.

domi sum - I am at home

domum eo - I go home

domo venio - I come from home

Those are the ones to learn as vocabulary rather than as grammar. They behave exactly like city names, which do the same thing: Romae, at Rome; Romam, to Rome; Roma, from Rome. Putting in or ad in front of any of them is the mark of someone applying the rules of the ablative and its prepositions where they do not apply.

What to take away

• Fourth declension: genitive -us, dative -ui, ablative -u, genitive plural -uum. Mostly masculine, mostly built off verbs.
• Manus and domus are feminine. Cornu and genu are neuter.
• Fifth declension: genitive -ei, ablative -e, genitive plural -rum. Mostly feminine, and only res and dies have a working plural.
• Dies is masculine except when it means a deadline or a period, where it is feminine.
• The nominative in -us proves nothing. The genitive proves everything.
• Domi, domum, domo are vocabulary, not grammar. Learn them as three fixed words.

Next: comparatives and superlatives, where Latin discovers that some things are taller than others and hands the whole system on to Spanish, French and Italian more or less intact.