Institor cum naviculario paciscitur.

Institor is the nominative singular, which is the form normally used for the subject of the sentence.

So in Institor cum naviculario paciscitur, institor is the person doing the action.

A quick case check:

• institor = nominative singular, the merchant / trader as subject
• institorem = accusative singular, would usually be a direct object
• institoris = genitive singular, of the merchant

Because the merchant is the subject here, the nominative institor is the right form.

The preposition cum means with, and it takes the ablative case.

So:

• navicularius = dictionary form, nominative singular
• naviculario = ablative singular

That is why Latin says cum naviculario = with the ship-owner / shipman / carrier by boat.

This is a very common pattern:

• cum amico = with a friend
• cum puella = with the girl
• cum naviculario = with the ship-owner

Because paciscitur is a deponent verb.

A deponent verb:

• has passive forms
• but an active meaning

So paciscitur looks passive, but it means something active like:

• he/she makes an agreement
• he/she bargains
• he/she negotiates

Its dictionary entry is usually:

• paciscor, pacisci, pactus sum

That combination tells you it is deponent.

So paciscitur is 3rd person singular present indicative, but from a deponent verb.

The subject is institor.

The verb ending -tur here tells you the verb is third person singular, so the subject must be he/she/it in the singular.

Since institor is nominative singular, it matches perfectly:

• institor = the merchant
• paciscitur = he/she bargains, makes a deal

So the sentence means that the merchant is the one making the agreement.

Not necessarily.

In this sentence, the verb is being used with the person with whom the agreement is made:

• cum naviculario = with the ship-owner

Very often with verbs of bargaining or agreeing, Latin can also express what is being agreed on, but it does not have to be stated every time.

So this sentence is complete as it stands:

• The merchant makes a deal with the ship-owner.

The exact thing being negotiated is simply left unstated.

Yes. Latin word order is much freer than English word order because the endings show the grammatical relationships.

So these could all mean essentially the same thing:

• Institor cum naviculario paciscitur.
• Cum naviculario institor paciscitur.
• Paciscitur institor cum naviculario.

However, the order can affect emphasis.

For example:

• putting cum naviculario earlier may emphasize with whom
• putting paciscitur first may emphasize the action itself

The version you have is a very normal, straightforward order.

Classical Latin does not have definite or indefinite articles like English the or a/an.

So:

• institor can mean merchant, a merchant, or the merchant
• naviculario can mean with a ship-owner or with the ship-owner

You figure out the best English translation from the context.

That is why one Latin sentence can often be translated into English in more than one natural way.

It is present tense.

More fully, it is:

• present
• indicative
• third person singular
• deponent

So it means:

• he/she bargains
• he/she is making an agreement
• he/she negotiates

Depending on context, English may translate the Latin present in slightly different ways.

Navicularius is a noun built from navicula (small ship, boat) and is used for someone connected with shipping or boat transport.

Depending on context, it can be translated as something like:

• ship-owner
• boatman
• shipper
• carrier by sea/river

In your sentence, the important grammar point is that its form is naviculario, the ablative singular, because it follows cum.

You work backward from the ending.

The ending -o can be ablative singular (or dative singular) for a second-declension masculine noun.

A learner would guess a nominative in -ius or -us, and here the dictionary form is:

• navicularius

So the forms are:

• nominative singular: navicularius
• ablative singular: naviculario

This is a very common pattern in second-declension nouns ending in -ius.

It is grammatically the same construction as ordinary accompaniment: cum + ablative.

But semantically, with paciscitur, it means the other party in the agreement.

So cum naviculario is not just together with the ship-owner in a physical sense. It means:

• with the ship-owner
• with the ship-owner as the person he is negotiating with

In other words, cum marks the person involved in the bargaining relationship.