Ventus adversus navem retinet.

Because of their case endings, not just their position.

• ventus is nominative singular, the case normally used for the subject.
• navem is accusative singular, the case normally used for the direct object.

So even if the word order changed, Latin would still show who is doing the action and who is receiving it.

No. In this sentence, adversus is best understood as an adjective, meaning something like adverse, opposing, or contrary.

So ventus adversus means an adverse wind or a contrary wind.

If adversus were a preposition here, navem would be its object, and then retinet would be left without a clear direct object. The sentence works much better if navem is the object of retinet.

Because adversus agrees with ventus in gender, number, and case.

• ventus = masculine, singular, nominative
• adversus = masculine, singular, nominative

But navem is feminine, singular, accusative. If the adjective described navem, you would expect a form like adversam, not adversus.

So agreement, not word order, shows which noun the adjective belongs to.

Because navem is the accusative singular form of navis.

• Dictionary form: navis = ship
• Accusative singular: navem = ship as a direct object

Since the ship is the thing being held back, Latin puts it in the accusative.

retinet is third-person singular present active indicative of retinere.

So it means he/she/it holds back, keeps back, or restrains.

Here the subject is ventus, so:

• ventus retinet = the wind holds back

You can also see the singular ending in -t, which matches the singular subject ventus.

Because Latin does not have articles like English the or a/an.

So:

• ventus can mean the wind or a wind
• navem can mean the ship or a ship

The context tells you which is more natural in translation.

Because Latin often places adjectives after the nouns they describe.

So ventus adversus is a normal Latin pattern:

• noun first
• adjective after it

Latin can also put adjectives before nouns, but noun + adjective is very common. English usually prefers adverse wind, while Latin is perfectly happy with wind adverse in its own word order.

Because Latin very often puts the verb near or at the end of the sentence.

So this pattern is very common:

• Ventus adversus = subject + adjective
• navem = object
• retinet = verb

Latin word order is more flexible than English because the endings already show each word’s role. The ending of navem tells you it is the object, even though it comes before the verb.

Not quite. In this sentence, retinet has the sense of holds back, checks, keeps from advancing, or delays.

So the idea is not just keeps in a general sense, but more specifically:

• the adverse wind holds the ship back
• the adverse wind prevents the ship from moving forward

That is why this verb fits so well with a ship and a contrary wind.