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This post is a draft translation from the Chinese version which have not yet been thoroughly proofread.

Introduction

Latin nouns are traditionally classified into five declensions according to their inflection patterns (adjectives generally use only the first three, while pronouns are usually irregular), as well as a few irregular nouns.

• Nominative: Indicates the subject of a sentence.
• Genitive: Indicates the processor.
• Dative: Indicates the recipient of the action, or the indirect object.
• Accusative: Indicates the object of a sentence.
• Ablative: Ablative in Latin has multiple functions:
  • Ablative of separation: 表示動作出發、離開的地方。視乎其修飾的詞彙，可能有也可能沒有介詞。抽象意義的分離不加介詞，具體意義的分離，且被修飾的詞彙不具有方位格時，添加介詞如 ab, ex, dē。
  • Ablative of place: 表示某事物所在的位置。用於條飾不具有方位格的詞彙時，添加介詞如 in, sub , prō。
  • Ablative of means/instrument: 表示行為所使用的工具。工具奪格前不加介詞。
  • Ablative of accompaniment: 介詞 cum 後加奪格表示伴隨。
• Vocative: Used to directly call on a person or thing.
• Locative: Indicates the position of a thing. Only some nouns have a locative form, other nouns express this meaning with a “preposition + ablative” structure.

General rules

• All endings of all declensions begin with a vowel, except some third declension words, and this vowel determines the word’s declension class. The differences between the various declension classes are simply the differences in these vowels (and some irregularities resulting from the evolution process). These vowels were originally part of the stem, but over a long period of evolution, they gradually merged with the ending and were subsequently reanalyzed as part of the ending.
• The nominative singular usually ends in either -s or null.
• The genitive singular and the nominative plural ends in either -ī or -s.
• The dative singular usually ends in -ī, except there a diphthong smoothing occurs in the second declension.
• Non-neuter accusative singulars end in -m, and the neuter ones are always identical to its nominative singular.
• The ablative singular ends in a vowel¹, which is a long vowel except in the third declension.
• The genitive plural ends in -um. The “long vowel + -rum” endings for the first, second and fifth declension are borrowed from pronoun paradigms.
• Non-neuter accusative plurals end in “long vowel + -s”.
• The dative/ablative plural ends in either -īs or -bus.
• The same form is used for both nominative and vocative, except in the second declension.
• The plurals of dative and ablative are identical for all nouns, with no exceptions.
• The nominative, accusative and vocative are identical for all neuter nouns, with an -a ending in their plurals, with no exceptions.

First & second declension

The stem final vowel of the first declension is a, and that of the second declension is o.

• The dative/ablative plural ending of these two declension classes were originally -ais and -ois respectively, later evolved to -īs in Classical Latin, which is a regular historical phonological process in Latin². Fīlia, fīliae and dea, deae have an irregular dative/ablative plural form ending in -ābus.
• The -us and -um endings in the second declension were -os and -om originally, the evolution to -us and -um in Classical Latin is also a regular phonological process³. The -os and -om endings are kept in monosyllabic pronouns.
• The -ō ending of the dative singular in the second declension was simplified from earlier -ōi.
• The -ī ending of the genitive singular in the second declension shuold be an irregular innovation. Before the time where -ei and -i started mixing up in Old Latin, the genitive singular was never written as -ei (even -osio was attested in Old Latin), showing that it did not come from -oi, which is different from the nominative plural.
• If a second declension noun ends in -ius or -ium in the nominative singular, its vocative singular would be -ī in Classical Latin (earlier -ie).

After the above back-reasoning process, the word forms of the two declensions will become extremely similar, differing only in the vowel at the beginning of the ending.

The second declension has a subclass, in which their nominative singular would have *-rus actually become -(e)r instead, such as ager — agrī, liber — librī, puer — puerī, vir — virī. These words have the same vocative singular and nominative singular, while the other inflections can be deduced from its genitive singular form, which is identical to all other second declension nouns.

Third declension

The third declension is a conflation of two originally distinct declension classes, one of which has a stem final vowel of i, while others have no stem final vowel. In Classical Latin, the two declension classes were almost mixed up entirely, their genitive plural remain distinct, being -um and -ium respectively.

The correspondence between the nominative singular and other word forms in the third declension has various types and is traditionally considered irregular. A decent amount of correspondence can actually be deduced from Latin’s (diachronical) sound rules, nevertheless:

Fourth & fifth declension

The stem final vowel of the fourth declension is u, and that of the fifth declension is ē.

The fourth and fifth declensions are the less used ones in Latin. Many words in these two declension classes were redistributed into the second or third declension during their development into Vulgar Latin or various Romance languages.

• The -ibus ending of the dative/ablative plural in the fourth declension came from earlier -ubus. A couple of words keep the -ubus ending, while the majority has undergone the rule of weakening word medial short vowels to i.

Irregular nouns

• Fīlia, fīliae and dea, deae have an irregular dative/ablative plural form ending in -ābus.
• The vocative singular of deus, deī is deus instead of *dee; Optional contracted forms exist for multiple hiatus-involving forms.
• Vīrus, vīrī is a second declension neuter noun with a citation form ending in -us (instead of -um). This word is an uncountable noun in Classical Latin. New Latin borrowed this word to express the meaning of “virus”, and a set of plural forms were made up by analogy to other second declension nouns.
• Domus, domūs/domī is a hybrid of second and fourth declension, with different mixing combination attested.

Adjectives

The declension of adjectives can be divided into the following categories:

• First/second declension adjectives: The masculine and neuter forms use the second declension, while the feminine forms use the first declension.
• Third declension adjectives: The nominative singular form can be further divided into three subclasses according to different genders, while other forms are similar to those of nouns in the third declension. Most (but not all) third declension adjectives have -ium in the genitive plural.
• • One termination: The nominative singular is identical in all three genders.
• • Two terminations: The nominative singular is identical in masculine and feminine, while a different form is used in neuter.
• • Three terminations: The nominative singular is different in all three genders. Usually -(e)r for masculine, -ris for feminine and -re for neuter.

Numerals