Latin words of the first declension have an invariable stem and are generally of feminine gender. The predominant letter in the ending forms of this declension is a. The nominative singular form consists of the stem and the affix-a, and the genitive singular form is the stem plus -ae.
There is a small category of masculine exceptions, which generally refer to occupations. These include 'farmer' (agricola, agricolae masc.), 'sailor' (nauta, nautae masc.), 'charioteer' (aurīga, aurīgae masc.), 'inhabitant' (incola, incolae masc.), 'pirate' (pīrāta, pīrātae masc.), 'writer' (scrība, scrībae masc.), and 'poet' (poēta, poētae masc).
The first declension also holds three types of Greek nouns, derived from Ancient Greek's Alpha Declension. They are declined irregularly in the singular. Occasionally, these Greek nouns may be declined as if they were native Latin nouns, e.g. nominative athlēta may be used instead of the original athlētēs.
The older genitive singular termination is an –ās. This is often used with familia as in pater familiās and māter familiās.
In poetry, the genitive singular –āī occurs. Aquae becomes aquāī.
The genitive plural ending –um replaces –ārum. This is actually a contraction. Puellum.
Because first declension nouns and second declension nouns display an –īs in the dative and ablative plural, words like equus (horse) and equa (mare) will end up looking alike in these cases. However, if a distinction must be made, equīs for 'mares' would become equābus in the dative and ablative plural. For this reason, the ending -ābus was regularly used in the dative and ablative plurals of the nouns dea (goddess) and filia (daughter).