Navis longa itineri apta est.

Because navis is in the nominative singular, which is the case normally used for the subject.

• navis = ship
• nominative singular of a 3rd-declension noun

So in Navis longa itineri apta est, navis is the thing being described as suitable.

Because longa has to agree with navis.

In Latin, adjectives agree with the nouns they describe in:

• gender
• number
• case

Here:

• navis is feminine
• singular
• nominative

So the adjective must also be feminine singular nominative:

• longa = long

If the noun were masculine, you might expect longus instead.

For the same reason: apta describes navis, so it must agree with it.

Even though apta is separated from navis by another word, it still matches:

• navis = feminine singular nominative
• apta = feminine singular nominative

So both longa and apta describe the same noun, navis.

Itineri is dative singular.

The adjective aptus, apta, aptum often takes the dative to mean:

• suitable for
• fit for
• adapted to

So:

• itineri = for a journey / for the journey

This is different from English, where we use for instead of a special case ending.

It comes from iter, meaning journey.

Its principal forms are:

• iter
• itineris

This is a 3rd-declension neuter noun. Its dative singular is itineri.

So:

• iter = journey
• itineri = to/for a journey

Classical Latin has no articles like English a, an, or the.

So navis can mean:

• a ship
• the ship

and itineri can mean:

• for a journey
• for the journey

You decide from context which English article is best.

Latin word order is much freer than English word order because the endings show how the words function.

So Navis longa itineri apta est could be rearranged in various ways without changing the basic meaning. For example:

• Navis longa apta est itineri
• Itineri navis longa apta est

The verb often comes at or near the end in Latin, but that is a tendency, not a rigid rule.

It is a normal noun phrase: navis longa = long ship.

Latin adjectives can come after the noun they modify, and very often do. So:

• navis longa literally has the order ship long
• but grammatically it means long ship

English usually puts adjectives before the noun; Latin often does not.

You know from agreement.

All three words match in:

• feminine
• singular
• nominative

So Latin is signaling that both adjectives belong with navis:

• navis = ship
• longa = long
• apta = suitable

The matching endings help you connect them, even when the word order is not what English speakers expect.

Sometimes Latin does omit forms of esse (to be), especially in poetry or very compressed style. But in a straightforward prose sentence like this, est is perfectly normal and expected.

So here:

• apta est = is suitable

Leaving out est would be possible only in certain contexts, not as the default for a beginner sentence.

No. apta is not a verb form here; it is an adjective.

So:

• apta = suitable
• est = is

Together they mean is suitable.

This is different from a passive form such as amatur (is loved), where the whole idea is expressed by one verb form. Here, Latin uses adjective + est.