Ex Ostia mercatrix Neapolim navigare vult, quia ibi amica eius habitat.

Ex means from or out of. It takes the ablative case, so Ostia is in the ablative here.

If macrons were written, you would normally see ex Ostiā. Since many beginner texts leave macrons out, Ostia can look the same as the dictionary form even though its case is different.

Latin has no articles. So mercatrix can mean the merchant or a merchant, depending on context, and amica can mean the friend or a friend.

English forces you to choose; Latin often does not.

Mercatrix is nominative singular, so it is the subject of vult.

A learner might expect the subject to be identified mainly by word order, as in English, but Latin usually shows grammatical function through case endings. Here mercatrix is the one doing the wanting.

Also, mercatrix is a feminine noun meaning female merchant.

Because Latin commonly uses volo, velle (to want) with an infinitive.

So:

• vult = she wants
• navigare = to sail

Together, navigare vult means she wants to sail.

This is very similar to English want + to + verb.

Latin often leaves subject pronouns out when they are already clear.

The ending of vult tells you it is third person singular: he/she/it wants. Since mercatrix is already there, Latin does not need to add ea for she.

So Latin often says simply:

• mercatrix vult = the merchant wants

instead of explicitly saying she wants.

With names of towns and cities, Latin often shows motion toward by using the accusative without a preposition.

So:

• Neapolim navigare = to sail to Naples

English needs to, but Latin often does not when the destination is a town.

Yes, it is a less familiar form. Neapolis is a Greek place name that Latin treats as a third-declension noun. Its accusative singular is Neapolim.

So the important point is:

• dictionary form: Neapolis
• motion toward the city: Neapolim

A beginner often expects something more like a first-declension ending, but this noun follows a different pattern.

Quia means because and introduces a reason clause.

So the structure is:

• main idea: mercatrix Neapolim navigare vult
• reason: quia ibi amica eius habitat

In other words, the second part explains why she wants to sail there.

Ibi means there. It refers to Neapolim—that is, in Naples.

So quia ibi amica eius habitat means because her friend lives there.

Ibi is an adverb, not a noun or adjective.

This is a very important Latin point.

Eius means his/her/its and is non-reflexive.
Suus, sua, suum is reflexive and normally refers back to the subject of its own clause.

In the clause quia ibi amica eius habitat, the subject is amica. So if you said amica sua, sua would refer back to amica, not to mercatrix.

That is why Latin uses eius here to mean her friend = the merchant’s friend.

Latin word order is much freer than English word order because Latin relies more on endings than on position.

English strongly prefers something like:

• The merchant wants to sail from Ostia to Naples

Latin can move pieces around more easily:

• Ex Ostia mercatrix Neapolim navigare vult

A few things are especially common in Latin:

• a setting or direction phrase can come early: Ex Ostia
• the main verb often comes late: vult
• the verb in a subordinate clause also often comes last: habitat

So the sentence may look unusual to an English speaker, but it is perfectly normal Latin.