Dux milites iubet urbem ab hostibus servare.

Because iubere works differently from English order.

In Latin, with iubere, the person being ordered is usually put in the accusative, not the dative:

• dux = the subject, the leader
• milites = the soldiers, the people being ordered

So Latin says, literally, something like:

The leader orders the soldiers to protect the city.

Even though English can feel as if the soldiers are the indirect object, Latin treats them as the direct object of iubet.

After iubet, Latin commonly uses an infinitive to express the action that is ordered.

So:

• iubet = orders
• servare = to protect / to save / to preserve

This is a very common Latin pattern:

someone + someone-accusative + infinitive

Here that gives:

• dux = the one giving the order
• milites = the ones ordered
• servare = what they are ordered to do

So servare is not a separate main verb. It depends on iubet.

They are both accusative, but they have different jobs.

• milites is the object of iubet and also the understood subject of servare
• urbem is the direct object of servare

So the structure is:

• Dux = subject of iubet
• milites = the soldiers whom the leader orders, and the ones who will do the protecting
• urbem = the thing being protected
• servare = the action

A helpful way to see it is:

The leader orders [the soldiers] [to protect the city].

Inside the infinitive idea to protect the city, the understood subject is milites.

Because dux is in the nominative singular, which is the normal case for the subject of a sentence.

Its form here is not very different from some other forms, so a beginner may hesitate, but the syntax makes it clear:

• dux = nominative singular, subject
• iubet = singular verb, so it matches dux
• milites and urbem are accusative, so they are not the main subject

So dux iubet means the leader orders.

Ab hostibus means from the enemies and uses:

• ab = from
• hostibus = ablative plural of hostis

So urbem ab hostibus servare means to protect the city from the enemies.

A very important point: this is not the agent of a passive verb here. Beginners often learn that ab + ablative can mean by with a passive, but this sentence is active, not passive.

So here ab hostibus means from the enemies, not by the enemies.

Because the enemies are not the direct object of the verb servare here.

The city is what is being protected:

• urbem = direct object, accusative

The enemies are introduced with ab to show separation or danger avoided:

• ab hostibus = from the enemies

So Latin is expressing:

protect the city from the enemies

not

protect the enemies.

Because the infinitive form itself already carries the idea of to.

So:

• servare = to protect, to save, to preserve

Latin does not need an extra word before the infinitive. English usually does, but Latin usually does not.

The word order is somewhat flexible, because the endings show the grammatical roles.

So all of these would still be understandable Latin, though with different emphasis:

• Dux milites iubet urbem ab hostibus servare
• Dux iubet milites urbem ab hostibus servare
• Urbem ab hostibus dux milites servare iubet

What matters most is the forms:

• dux = nominative
• milites = accusative plural
• urbem = accusative singular
• hostibus = ablative plural

That said, the given order is quite natural and easy to follow.

All three are possible basic meanings of servare, depending on context.

Common possibilities include:

• save
• protect
• preserve
• keep safe

In this sentence, because of urbem ab hostibus, protect or save is probably the most natural choice in English.

So a learner should understand that servare has a wider range than just one fixed English word.

Because Latin has no articles.

So:

• dux can mean leader, a leader, or the leader
• urbem can mean city, a city, or the city
• hostibus can mean enemies, the enemies, or sometimes some enemies, depending on context

English must choose a or the, but Latin usually leaves that to context.

Yes. A very literal unpacking can help:

The leader orders the soldiers to protect the city from the enemies.

Or even more mechanically:

Leader soldiers orders city from enemies to protect.

That second version is not good English, but it can help you see how the Latin pieces fit together:

• Dux = leader
• milites = soldiers
• iubet = orders
• urbem = city
• ab hostibus = from enemies
• servare = to protect

This kind of unpacking is often useful when learning Latin sentence structure.