The Stem of nouns of the Fourth Declension ends in u-. This is usually weakened to i before -bus. Masculine and Feminine nouns form the nominative by adding s; Neuters have for nominative the simple stem, but with ū (long).
FOURTH DECLENSION
§§88–94 · 7 sections
Nouns of the Fourth Declension are declined as follows:
| manus, F., hand | lacus, M., lake | genū, N., knee |
| STEM manu- | STEM lacu- | STEM genu- |
| SINGULAR | |||||
| CASE-ENDINGS | CASE-ENDINGS | ||||
| NOM. | manus | lacus | -us | genū | -ū |
| GEN. | manūs | lacūs | -ūs | genūs | -ūs |
| DAT. | manuī(-ū) | lacuī(-ū) | -uī(-ū) | genū | -ū |
| ACC. | manum | lacum | -um | genū | -ū |
| ABL. | manū | lacū | -ū | genū | -ū |
| PLURAL | |||||
| NOM. | manūs | lacūs | -ūs | genua | -ua |
| GEN. | manuum | lacuum | -uum | genuum | -uum |
| DAT. | manibus | lacubus | -ibus(-ubus) | genibus | -ibus |
| ACC. | manūs | lacūs | -ūs | genua | -ua |
| ABL. | manibus | lacubus | -ibus(-ubus) | genibus | -ibus |
Most nouns of the Fourth Declension in -us are Masculine.
Exceptions: The following are Feminine: acus, anus, colus, domus, īdūs (plural), manus, nurus, porticus, quīnquātrūs (plural), socrus, tribus, with a few names of plants and trees. Also, rarely, penus, specus.
The only Neuters of the Fourth Declension are cornū, genū, pecū (§ 105. f), verū.
Case-Forms in the Fourth Declension
The following peculiarities in case-forms of the Fourth Declension require notice:—
A genitive singular in -ī (as of the second declension) sometimes occurs in nouns in -tus: as, senātus, genitive senātī (regularly senātūs).
In the genitive plural -uum is sometimes pronounced as one syllable, and may then be written -um: as, currum
(Aen. 6.653) for curruum.
The dative and ablative plural in -ŭbus are retained in partus and tribus; so regularly in artus and lacus, and occasionally in other words; portus and specus have both -ubus and -ibus.
Most names of plants and trees, and colus, distaff, have also forms of the second declension: as, fīcus, fig, genitive fīcūs or fīcī.
An old genitive singular in -uis or -uos and an old genitive plural in -uom occur rarely: as, senātuis, senātuos; fluctuom.
The ablative singular ended anciently in -ūd (cf. § 43. N. 1): as, magistrātūd.
Domus (F.), house, has two stems ending in u- and o-. Hence it shows forms of both the fourth and second declensions:
| SINGULAR | PLURAL | |
| NOM. | domus | domūs |
| GEN. | domūs (domī, loc.) | domuum (domōrum) |
| DAT. | domuī (domō) | domibus |
| ACC. | domum | domōs (domūs) |
| ABL. | domō (domū) | domibus |
Most nouns of the Fourth Declension are formed from verb-stems, or roots, by means of the suffix -tus (-sus) (§ 238. b):
- cantus, song, CAN, canō, sing; cāsus (for †cad-tus), chance, CAD, cadō, fall, exsulātus, exile, from exsulō, to be an exile (exsul).
Many are formed either from verb-stems not in use, or by analogy:
- cōnsulātus (as if from †cōnsulō, -āre), senātus, incestus.
The accusative and the dative or ablative of nouns in -tus (-sus) form the Supines of verbs (§ 159. b): as, spectātum, petītum; dictū, vīsū.
Of many verbal derivatives only the ablative is used as a noun: as, iussū (meō), by (my) command; so iniussū (populī), without (the people's) order. Of some only the dative is used: as, dīvīsuī.