Dum nautae signa tempestatis in caelo observant, mercator e portu discedere non vult.

Here dum means while. It introduces a temporal subordinate clause:

dum nautae signa tempestatis in caelo observant
= while the sailors are observing signs of a storm in the sky

So the dum clause gives the time during which the action of the main clause happens.

Yes. Latin observant is present tense, and the Latin present can often be translated in more than one way in English:

• they observe
• they are observing
• sometimes even they do observe, depending on context

So here observant can naturally mean are observing because the action is going on at the same time as the merchant’s unwillingness to leave.

Here nautae is nominative plural, and it is the subject of observant.

So:

• nautae = the sailors
• observant = observe / are observing

Together: the sailors are observing

A learner may notice that nautae could have other forms in other contexts, but here the verb observant is plural, so nautae is clearly the plural subject.

signa means signs, and tempestatis is genitive singular of tempestas, meaning storm or bad weather.

So:

• signa = signs
• tempestatis = of a storm

Together, signa tempestatis means signs of a storm.

The genitive here shows a relationship like of in English. This is very common in Latin.

signa is accusative plural because it is the direct object of observant.

The sailors are doing the action of observing, and the thing they observe is signa tempestatis.

So:

• subject: nautae
• verb: observant
• direct object: signa tempestatis

in caelo means in the sky.

Here in takes the ablative because it expresses location:

• in + ablative = in / on a place
• in + accusative = into / onto a place, showing motion toward

Since the signs are being observed in the sky, not moving into the sky, Latin uses the ablative: caelo.

The main clause is:

mercator e portu discedere non vult

This is the part that could stand on its own as a complete statement. The dum clause is subordinate and gives the time.

So the structure is:

• subordinate clause: dum nautae signa tempestatis in caelo observant
• main clause: mercator e portu discedere non vult

Because vult means wants, and after verbs like volo in Latin, the action wanted is usually expressed by an infinitive.

So:

• vult = he wants
• discedere = to leave / depart

Together:

discedere vult = he wants to leave
discedere non vult = he does not want to leave

This is a very common Latin pattern:

• potest facere = he can do
• debet venire = he ought to come
• vult discedere = he wants to leave

portu is ablative singular of portus, and it follows the preposition e meaning out of or from.

So:

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

Many Latin prepositions regularly take a specific case. e/ex takes the ablative.

Both e and ex mean the same thing here: out of / from. Latin uses both forms.

A common tendency is:

• e often before a consonant
• ex often before a vowel or h

But this is not a rigid rule in every text. In this sentence, e portu is perfectly normal.

Non negates the verb vult:

• vult = wants
• non vult = does not want

So discedere non vult means does not want to leave.

English often puts the negative with do in this kind of sentence, but Latin simply puts non before the verb being negated.

Yes, this is very normal Latin word order.

Latin word order is more flexible than English because the endings show the grammatical relationships. Still, Latin often likes to place verbs near the end of their clauses.

So we get:

• dum ... observant
• mercator ... discedere non vult

Also, putting the dum clause first is natural because it sets the scene: while the sailors are watching for signs of a storm...

Because they belong to different clauses and have different verbs.

• nautae is the subject of observant
• mercator is the subject of vult

So the sentence is saying:

• the sailors are observing
• the merchant does not want to leave

There is no requirement that both subjects have the same number. Latin handles each clause separately.