Cum omen bonum esset, nautae e portu exire volebant.

Latin cum can be either:

• a preposition meaning with
  • ablative
• a conjunction meaning when, since, or although

Here it is a conjunction because it introduces a whole clause: cum omen bonum esset.

So:

• cum + noun in the ablative = with
• cum + verb/clause = when / since / although

In this sentence, cum introduces the circumstance under which the sailors wanted to leave.

After cum, Latin often uses the subjunctive when the clause gives the circumstances/background of the main action. This is often called a circumstantial cum clause.

So:

• cum omen bonum erat would be a more straightforward when the omen was good
• cum omen bonum esset gives the sense of since / when the omen was favorable, under those circumstances

This is a very common construction in Latin narrative.

So the subjunctive here is not mainly about doubt. It is there because cum often takes the subjunctive when introducing a background clause.

Esset is the imperfect subjunctive of sum, esse.

It is used because the main verb is also in a past tense: volebant (they were wanting / wanted).

Latin often matches past-time background with the imperfect subjunctive in a cum clause:

• cum ... esset
• main verb: volebant

This shows that the good omen was the situation or background at the time when the sailors wanted to leave.

Because omen is a neuter noun.

Its adjective must agree with it in:

• gender
• number
• case

So:

• omen = neuter singular nominative
• bonum = neuter singular nominative

That is why Latin uses bonum, not bonus.

Omen is nominative singular.

It is the subject of esset:

• omen bonum esset = the omen was favorable / good

Even though English sometimes uses it was a good omen, Latin here is structured as the omen was good.

Nautae here is nominative plural, meaning the sailors.

It is the subject of volebant:

• nautae ... volebant = the sailors wanted

This noun is a good one to notice because nauta, nautae is a first-declension noun, but it refers to a male person. That can feel unusual to English speakers, who may expect grammatical patterns to match natural gender more closely.

Portu is ablative singular.

The preposition e/ex means out of or from, and it takes the ablative case.

So:

• e portu = out of the harbor / from the harbor

This is a standard Latin pattern:

• e/ex + ablative

Both e and ex mean the same thing here: out of / from.

Latin often chooses between them partly for sound:

• ex is especially common before vowels and sometimes before certain consonants
• e is often used before consonants

Since portu begins with p, e portu sounds perfectly natural.

In many cases, either form may be possible, but e portu is the expected simpler form here.

Because it depends on volebant.

Volo means want, and after it Latin normally uses an infinitive to say what someone wants to do.

So:

• volebant exire = they wanted to leave

This is like English:

• they wanted to leave not
• they wanted they left

So exire is the infinitive that completes the meaning of volebant.

Exire is the present active infinitive of exeo, exire, meaning to go out, to leave, or to depart.

It is a compound of:

• ex- = out
• eo, ire = to go

So exire literally means to go out.

In this sentence, with e portu, it means to leave the harbor or to sail out of the harbor.

Yes, both contain the idea of out, but they do different jobs.

• exire tells you the basic action: to go out / leave
• e portu tells you the place being left: out of the harbor

So the phrase is not redundant in a bad way. It is more like:

• to depart from the harbor
• to go out of the harbor

Latin often combines a compound verb with a prepositional phrase like this.

Volebant is the imperfect tense of volo.

It can mean:

• they wanted
• they were wanting

In smoother English, they wanted is usually best here.

The imperfect often gives a sense of an ongoing or background action in the past. So the sentence presents the sailors’ desire to leave as something happening at that time, not as a single completed action.

Latin word order is more flexible than English because the endings show the grammatical relationships.

This sentence is:

• Cum omen bonum esset, nautae e portu exire volebant.

A very literal order might be:

• When the omen favorable was, the sailors from the harbor to leave wanted.

Latin can do this because:

• esset clearly goes with omen
• nautae is clearly the subject of volebant
• e portu is marked by the preposition and ablative
• exire is an infinitive depending on volebant

The word order also helps emphasis and style. Latin often places the verb at or near the end of the clause.

Yes, depending on context, cum with the subjunctive can sometimes be translated as since, when, or even although.

Here the most natural translation is probably when or since, depending on what the larger passage is trying to emphasize:

• when the omen was favorable...
• since the omen was favorable...

If the sentence is just giving the circumstances, when is likely best. If it is giving a reason, since may fit better.

That is one reason Latin cum clauses can feel slightly flexible in translation.

Literally, bonum means good.

But with omen, English often prefers favorable, because that sounds more natural:

• a good omen
• the omen was favorable

So bonum is literally good, but favorable is often the better idiomatic translation in context.