Ultima navis portum relinquit.

Because it agrees with navis.

In Latin, adjectives must match the noun they describe in:

• gender
• number
• case

Here, navis is:

• feminine
• singular
• nominative (the subject)

So the adjective appears as ultima, the feminine singular nominative form of ultimus, -a, -um.

So:

• ultima navis = the last ship

There are two main clues:

1. relinquit is a singular verb, so the subject should be singular.
2. navis is in the nominative singular, which is the normal case for the subject.

So navis is the thing doing the action:

• navis relinquit = the ship leaves

By contrast, portum is in the accusative, so it is the direct object.

Because it is the direct object of relinquit.

The verb relinquere means to leave behind, to depart from, or simply to leave something. The thing being left takes the accusative.

So:

• portus = harbor, port (dictionary form, nominative)
• portum = harbor/port as the direct object, accusative singular

In this sentence:

• Ultima navis portum relinquit = The last ship leaves the harbor

Relinquit is:

• 3rd person singular
• present tense
• active voice
• indicative mood

It comes from relinquere.

So it means:

• he/she/it leaves
• in this sentence, it leaves, referring to navis

Because navis is singular, the verb is singular too.

Latin has no definite article and no indefinite article.

So Latin does not normally use separate words for:

• the
• a/an

Whether you translate navis as ship, a ship, or the ship depends on context.

So:

• ultima navis can naturally be translated as the last ship
• but literally it is just last ship

English requires an article; Latin usually does not.

It matters, but not in the same rigid way as in English.

Latin uses endings to show grammatical function, so word order is more flexible. Here:

• navis is the subject because of its form
• portum is the object because of its form

So even if you changed the order, the basic meaning would still be clear, for example:

• Portum ultima navis relinquit
• Relinquit ultima navis portum

All of these can mean The last ship leaves the harbor.

That said, word order can affect emphasis:

• putting ultima or navis first may highlight the ship
• putting portum first may highlight the harbor
• putting relinquit last is very normal in Latin prose

Navis is a third-declension noun, even though it is feminine.

Its dictionary form is:

• navis, navis = ship

Many learners first associate:

• feminine nouns with first declension
• masculine/neuter nouns with other declensions

But in Latin, gender and declension are different things. A noun can be feminine without being first declension.

So:

• navis = feminine
• third declension

Its nominative singular happens to end in -is, not -a.

Portus is a fourth-declension masculine noun.

Its dictionary form is:

• portus, -us = harbor, port

In the fourth declension, the accusative singular masculine often ends in -um.

So:

• portus = nominative singular
• portum = accusative singular

This can be confusing because -um is also common in second-declension neuter nominatives and accusatives. But here portum is specifically the accusative singular of a fourth-declension masculine noun.

Usually in a sentence like this, ultima means last.

But the adjective ultimus, -a, -um can also have related meanings depending on context, such as:

• furthest
• final
• uttermost
• endmost

So in another context, ultima navis could theoretically mean something like:

• the furthest ship
• the final ship

But in an ordinary sentence about ships leaving a harbor, the last ship is the most natural meaning.

Not in this exact form.

If navis were the direct object, you would normally expect the accusative singular:

• navem

Compare:

• navis = nominative singular, the ship as subject
• navem = accusative singular, the ship as object

So in this sentence:

• navis must be the subject
• portum must be the object

In a reconstructed Classical pronunciation, approximately:

OOL-ti-ma NA-wis POR-tum re-LIN-kwit

A few notes:

• v is pronounced like English w
• c and g are always hard, though there is no c here
• qu is pronounced like kw
• stress usually falls on:
  • ULTima
  • NĀvis? More exactly NAvis
  • PORtum
  • reLINquit

So the sentence sounds roughly like:

OOL-ti-ma NA-wis POR-tum re-LIN-kwit

Because the verb agrees with the subject, not with the total number of words.

The subject here is navis, which is singular:

• navis = ship, one ship

Ultima is just an adjective modifying navis, not a separate subject.

So:

• ultima navis = one subject
• therefore relinquit = singular

If the subject were plural, the verb would also be plural, for example:

• Ultimae naves portum relinquunt = The last ships leave the harbor

Either can work, depending on context.

Latin present tense often covers both:

• simple present: leaves
• progressive present: is leaving

So relinquit can mean:

• leaves
• is leaving

In a basic vocabulary-and-grammar setting, leaves is the usual translation. But in a real context, is leaving may sometimes sound more natural.