Gender-specific

• For pure Latin neuter nouns, the nominative, vocative, and accusative cases are identical, and the nominative, vocative, and accusative plurals all end in -a. (Both of these features are inherited from Proto-Indo-European, and so no actual syncretism is known to have happened in the historical sense, since these cases of these nouns are not known to have ever been different in the first place.)

Case-specific

• The vocative form is always the same as the nominative in the plural, and usually the same as the nominative in the singular except for second-declension nouns ending in -us and a few nouns of Greek origin. For example, the vocative of the first-declension Aenēās is Aenēā.
• The genitive singular is the same as the nominative plural in first-, second-, and fourth-declension non-neuter Latin nouns.
• The dative singular is the same as the genitive singular in first- and fifth-declension pure Latin nouns.
• The dative is always the same as the ablative in the singular in the second declension, the third-declension full i-stems (i.e. neuter i-stems, adjectives), and fourth-declension neuters.
• The dative, ablative, and locative are always identical in the plural.
• The locative is identical to the ablative in the 3rd, 4th and 5th declensions.
• The nominative and accusative plurals are identical in the athematic declensions, the masculine/feminine ending in -ēs (3rd/5th declension) or -ūs (4th declension).

First declension Greek nouns

The first declension also includes three types of Greek loanwords, derived from Ancient Greek's alpha declension. They are declined irregularly in the singular, but sometimes treated as native Latin nouns, e.g. nominative athlēta ('athlete') instead of the original athlētēs. Archaic (Homeric) first declension Greek nouns and adjectives had been formed in exactly the same way as in Latin: nephelēgeréta Zeus ('Zeus the cloud-gatherer') had in classical Greek become nephelēgerétēs.

For full paradigm tables and more detailed information, see the Wiktionary appendix First declension.

Second-declension -ius and -ium nouns

Nouns ending in -ius and -ium have a genitive singular in -ī in earlier Latin, which was regularized to -iī in the later language. Masculine nouns in -ius have a vocative singular in -ī at all stages. These forms in -ī are stressed on the same syllable as the nominative singular, sometimes in violation of the usual Latin stress rule. For example, the genitive and vocative singular Vergilī (from Vergilius) is pronounced Vergílī, with stress on the penult, even though it is short.^[6] In Old Latin, however, the vocative was declined regularly, using -ie instead, e.g. fīlie "[O] son", archaic vocative of fīlius.

There is no contraction of -iī(s) in plural forms and in the locative.

In the older language, nouns ending with -vus, -quus and -vum take o rather than u in the nominative and accusative singular. For example, servus, servī ('slave') could be servos, accusative servom.

Second-declension -r nouns

Some masculine nouns of the second declension end in -er or -ir in the nominative singular. The declension of these nouns is identical to that of the regular second declension, except for the lack of suffix in the nominative and vocative singular.

Some (but not all) nouns in -er drop the e genitive and other cases. For example, socer, socerī ('father-in-law') keeps its e. However, the noun magister, magistrī ('(school)master') drops its e in the genitive singular.

For declension tables of second-declension nouns, see the corresponding Wiktionary appendix.

The vocative puere is found but only in Plautus.^[7] The genitive plural virum is found in poetry.^[8]

Second-declension Greek nouns

The second declension contains two types of masculine Greek nouns and one form of neuter Greek noun. These nouns are irregular only in the singular, as are their first-declension counterparts. Greek nouns in the second declension are derived from the Omicron declension.

Some Greek nouns may also be declined as normal Latin nouns. For example, theātron can appear as theātrum.

Irregular forms

Consonant stems

The stem of a consonant-stem noun may be found from the genitive case by removing the ending -is. For example, the stem of pāx, pācis f. 'peace' is pāc-, the stem of flūmen, flūminis n. 'river' is flūmin-, and the stem of flōs, flōris m. 'flower' is flōr-.

Masculine, feminine and neuter nouns often have their own special nominative singular endings. For instance, many masculine nouns end in -or (amor, amōris, 'love'). Many feminine nouns end in -īx (phoenīx, phoenīcis, 'phoenix'), and many neuter nouns end in -us with an r stem in the oblique cases (onus, oneris 'burden'; tempus, temporis 'time').

The locative endings for the third declension are -ī or -e (singular) and -ibus (plural), as in rūrī 'in the country' and Trallibus 'at Tralles'.^[14]

Third declension i-stem and mixed nouns

The third declension also has a set of nouns that are declined differently. They are called i-stems. i-stems are broken into two subcategories: pure and mixed. Pure i-stems are indicated by special neuter endings. Mixed i-stems are indicated by the double consonant rule. Stems indicated by the parisyllabic rule are usually mixed, occasionally pure.

Masculine and feminine

Parisyllabic rule: Some masculine and feminine third-declension i-stem nouns have the same number of syllables in the genitive and the nominative. For example: nāvis, nāvis ('ship'); nūbēs, nūbis ('cloud'). The nominative ends in -is or -ēs.

Double consonant rule: The rest of the masculine and feminine third-declension i-stem nouns have two consonants before the -is in the genitive singular. For example: pars, partis ('part').

Neuter

Special neuter ending: Neuter third-declension i-stems have no rule. However, all of them end in -al, -ar or -e. For example: animal, animālis ('animal'); cochlear, cochleāris ('spoon'); mare, maris ('sea').

The mixed declension is distinguished from the consonant type only by having -ium in the genitive plural (and occasionally -īs in the accusative plural). The pure declension is characterized by having -ī in the ablative singular, -ium in the genitive plural, -ia in the nominative and accusative plural neuter, and -im in the accusative singular masculine and feminine (however, adjectives have -em).

The accusative plural ending -īs is found in early Latin up to Virgil, but from the early empire onwards it was replaced by -ēs.^[15]

The accusative singular ending -im is found only in a few words: always in tussis 'cough', sitis 'thirst', Tiberis 'River Tiber'; usually in secūris 'axe', turris 'tower', puppis 'poop', febris 'fever'; occasionally in nāvis 'ship'. Most nouns, however, have accusative singular -em.^[16]

The ablative singular -ī is found in nouns which have -im, and also, optionally, in some other nouns, e.g. in ignī or in igne 'in the fire'.

There are two mixed-declension neuter nouns: cor, cordis ('heart') and os, ossis ('bone'). Also, the mixed declension is used in the plural-only adjective plūrēs, plūra ('most').

The rules for determining i-stems from non-i-stems and mixed i-stems are guidelines rather than rules: many words that might be expected to be i-stems according to the parisyllabic rule actually are not, such as canis ('dog') or iuvenis ('youth'), which have genitive plural canum 'of dogs' and iuvenum 'of young men'. Likewise, pater ('father'), māter ('mother'), frāter ('brother'), and parēns ('parent') violate the double-consonant rule. This fluidity even in Roman times resulted in much more uncertainty in Medieval Latin.

Some nouns in -tāt-, such as cīvitās, cīvitātis 'city, community' can have either consonant-stem or i-stem genitive plural: cīvitātum or cīvitātium 'of the cities'.^[15]

Peculiarities

In the third declension, there are four irregular nouns.

Domus

Domus ('house, dwelling, building, home, native place, family, household, race') is an irregular noun, mixing fourth and second declension nouns at the same time (especially in literature). However, in practice, it is generally declined as a regular -us stem fourth declension noun (except by the ablative singular and accusative plural, using -ō and -ōs instead).^[17]

Possessive pronouns' declensions

The possessive adjective vester has an archaic variant, voster; similar to noster.

Usually, to show the ablative of accompaniment, cum would be added to the ablative form. However, with personal pronouns (first and second person), the reflexive and the interrogative, -cum is added onto the end of the ablative form. That is: mēcum 'with me', nōbīscum 'with us', tēcum 'with you', vōbīscum, sēcum and quōcum (sometimes quīcum).

Pronouns have also an emphatic form bi using the suffix -met (egomet, tūte/tūtemet, nosmet, vosmet), used in all cases, except by the genitive plural forms.

In accusative case, the forms mēmē and tētē exist as emphatic, but they are not widely used.

Sē, suī has a possessive adjective: suus, sua, suum, meaning 'his/her/its/their own':

Patrem suum numquam vīderat. (Cicero)^[19]

"He had never seen his [own] father."

When 'his' or 'her' refers to someone else, not the subject, the genitive pronoun eius (as well as eōrum and eārum) 'of him' is used instead of suus:

Fit obviam Clodiō ante fundum eius. (Cicero)^[20]

"He met Clodius in front of the latter's farm."

When one sentence is embedded inside another with a different subject, sē and suus can refer to either subject:

Patrēs conscrīptī ... lēgātōs in Bīthȳniam miserunt quī ab rēge peterent, nē inimīcissimum suum secum haberet sibique dēderet. (Nepos)^[21]

"The senators ... sent ambassadors to Bithynia, who were to ask the king not to keep their greatest enemy with him but hand him over to them."

For the third-person pronoun is 'he', see below.

First- and second-declension -r adjectives

Some first- and second-declension adjectives' masculine forms end in -er. As with second-declension -r nouns, some adjectives retain the e throughout inflection, and some omit it. Sacer, sacra, sacrum omits its e while miser, misera, miserum keeps it.

First and second declension pronominal adjectives

Nine first and second declension pronominal adjectives are irregular in the genitive and the dative in all genders. They can be remembered by using the mnemonic acronym ūnus nauta. They are:

• ūllus, ūlla, ūllum 'any';
• nūllus, nūlla, nūllum 'no, none';
• uter, utra, utrum 'which [of two], either';
• sōlus, sōla, sōlum 'sole, alone';
• neuter, neutra, neutrum 'neither';
• alius, alia, aliud 'another' (the genitive singular alīus is often replaced by alterīus or by aliēnus 'of another');^[22]
• ūnus, ūna, ūnum 'one';
• tōtus, tōta, tōtum 'whole';
• alter, altera, alterum 'other [of two]'.

Third-declension adjectives with one ending

These have a single nominative ending for all genders, although as usual the endings for the other cases vary. As with nouns, a genitive is given for the purpose of showing the inflection.

Third-declension adjectives with two endings

Third-declension adjectives that have two endings have one form for the masculine and feminine, and a separate form for the neuter. The ending for the masculine and feminine is -is, and the ending for the neuter is -e. It is not necessary to give the genitive, as it is the same as the nominative masculine singular.

Third-declension adjectives with three endings

Third-declension adjectives with three endings have three separate nominative forms for all three genders. Like third and second declension -r nouns, the masculine ends in -er. The feminine ends in -ris, and the neuter ends in -re. The genitive is the same as the nominative feminine singular.

General pattern for comparatives

Comparatives and superlatives with normal endings

Comparatives and superlatives of -er adjectives

Adjectives (in the first and second as well as third declensions) that have masculine nominative singular forms ending in -er are slightly different. As with normal adjectives, the comparative is formed by adding -ior to the stem, but for the superlative, -rimus is added to the nominative masculine singular.

Comparatives and superlatives of -lis adjectives

Some third declension adjectives with two endings in -lis in the masculine–feminine nominative singular have irregular superlative forms. The following are the only adjectives that do.

Comparatives and superlatives of -eus/-ius adjectives

First and second declension adjectives that end in -eus or -ius are unusual in that they do not form the comparative and superlative by taking endings at all. Instead, magis ('more') and maximē ('most'), the comparative and superlative degrees of magnoperē ('much, greatly'), respectively, are used.

Many adjectives in -uus, except those in -quus or -guus, also follow this rule.

Irregular comparatives and superlatives

As in most languages, Latin has adjectives that have irregular comparatives and superlatives.

Cardinal numerals

All cardinal numerals are indeclinable, except ūnus ('one'), duo ('two'), trēs ('three'), plural hundreds ducentī ('two hundred'), trecentī ('three hundred') etc., and mīlle ('thousand'), which have cases and genders like adjectives. Ūnus, ūna, ūnum is declined like a first- and second-declension pronoun with -īus or -ius in the genitive, and -ī in the dative. Duo is declined irregularly, trēs is declined like a third-declension plural adjective, -centī ('hundred') numerals decline like first- and second-declension adjectives, and mīlle is invariable in the singular and declined like a third-declension i-stem neuter noun in the plural:

The plural endings for ūnus are used with plūrālia tantum nouns, e. g. ūna castra (one [military] camp), ūnae scālae (one ladder).

The word ambō ('both'), is declined like duo except that its o is long. Both declensions derive from the Indo-European dual number, otherwise defunct in Latin, rather than the plural.

The numeral centum ('one hundred') is indeclinable, but all the other hundred numerals are declinable (ducentī, trecentī, quadringentī, quīngentī, sescentī, septingentī, octingentī, nōngentī).

The word mīlle 'thousand' is a singular indeclinable adjective. However, its plural, mīlia, is a plural third-declension i-stem neuter noun. To write the phrase "four thousand horses" in Latin, the genitive is used: quattuor mīlia equōrum, literally, "four thousands of horses".

The rest of the numbers are indeclinable whether used as adjectives or as nouns.

For further information on the different sets of Latin numerals, see Latin numerals (linguistics).