Si hostes iterum oppugnabunt, cives et milites una urbem defendent.

This is a very common question. In Latin, a simple future condition often uses the future indicative in both clauses:

• Si hostes iterum oppugnabunt = If the enemies attack / will attack again
• cives et milites una urbem defendent = the citizens and soldiers will defend the city together

So Latin can say si ... oppugnabunt where English normally prefers if ... attack. The Latin future makes the future time very explicit.

Si means if and introduces the condition.

So the sentence has two parts:

• Si hostes iterum oppugnabunt = the if-clause (the condition)
• cives et milites una urbem defendent = the main clause (what will happen if the condition is met)

This is the basic Latin way to build a conditional sentence.

Here hostes is nominative plural, because it is the subject of oppugnabunt.

So:

• hostes = the enemies
• oppugnabunt = they will attack

Together: the enemies will attack.

A useful extra point: hostes can also be accusative plural in other contexts, because some third-declension forms look the same in nominative and accusative plural. Here the verb and sentence structure show that it must be the subject, so it is nominative.

Because urbem is the direct object of defendent.

Latin uses the accusative case for a direct object, so:

• urbs = city as a subject form
• urbem = city as an object form

Since the citizens and soldiers are defending the city, city is the thing being defended, so Latin uses urbem.

Both are nominative plural, because together they form the subject of defendent.

• cives = citizens
• milites = soldiers

Linked by et, they make a compound subject:

• cives et milites = the citizens and soldiers

These are the people who will do the defending.

Here una means together or as one.

It is being used as an adverb, modifying defendent:

• una defendent = they will defend together

This is not the adjective meaning one in this sentence. Context shows that it means together.

If you learn with macrons, you may sometimes see the adverb written ūnā, but many texts leave macrons out.

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

English depends heavily on order:

• The soldiers defend the city is different from The city defends the soldiers

Latin can move words around more freely because the cases and verb endings make the roles clear:

• cives et milites are nominative plural subjects
• urbem is accusative singular object
• defendent is the verb

So this sentence could be rearranged in other ways and still mean the same basic thing, though the emphasis might change.

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

So words like:

• hostes
• cives
• milites
• urbem

can mean the enemies, the citizens, the soldiers, the city, or sometimes enemies, citizens, soldiers, a city, depending on context.

English has to add articles when translating, but Latin usually leaves that idea to context.

Because the two verbs belong to different conjugations, and Latin forms the future differently depending on the conjugation.

• oppugnare is a 1st-conjugation verb

  • future: oppugnabo, oppugnabis, oppugnabit, oppugnabimus, oppugnabitis, oppugnabunt

• defendere is a 3rd-conjugation verb

  • future: defendam, defendes, defendet, defendemus, defendetis, defendent

So:

• oppugnabunt = they will attack
• defendent = they will defend

This is one of the first big future-tense patterns Latin learners have to get used to.

Iterum is an adverb meaning again.

It modifies oppugnabunt:

• hostes iterum oppugnabunt = the enemies will attack again

Like many adverbs in Latin, it can move around more freely than in English. Its position here is natural, but Latin word order is not as fixed as English word order.

Because the verb ending already tells you the subject is third person plural: they.

• oppugnabunt = they will attack
• defendent = they will defend

So Latin usually does not need to add a separate pronoun unless it wants emphasis or contrast.

In this sentence, the nouns themselves already identify who they are:

• hostes for oppugnabunt
• cives et milites for defendent