There is no distinction between "the" and "a/an" in this example. These words could very easily refer to "a girl," or "girls" in general when using the plural.
Despite most of the nouns in the first declension being feminine, some nouns denoting a traditionally masculine role are treated as masculine. Examples would be words like nauta (sailor) or pirata (pirate).
Odds and Ends
- In poetry, you can find the genetive form -āīM in place of the regular -ae.
- The Locative singular is the same as the dative, -ae. Example: Romae, at Rome.
- Words like filia (daughter) and dea (goddess) have different dative and ablative plural inflections, which are -abus instead of -īs. This is done to avoid having similar inflections as their masculine counterparts, which would have the same inflections.
Rule of thumb — If a first declension feminine noun has a masculine counterpart in the second declension (i.e. it has the same root), use the -abus ending for dative and ablative plural inflections.