Postquam institor merces aestimavit, de naulo pacti sunt.

Because Latin often uses the perfect indicative after postquam to describe an action that happened before the next one in the story.

So here:

• Postquam institor merces aestimavit = After the merchant appraised the goods
• de naulo pacti sunt = they agreed about the freight charge

In English, we sometimes use had appraised to make the earlier action clearer, but Latin very commonly just uses the perfect: aestimavit.

Institor usually means a dealer, trader, shopkeeper, or merchant.

It is a masculine noun, and here it is in the nominative singular, so it is the subject of aestimavit:

• institor = the merchant
• aestimavit = appraised / estimated

So institor merces aestimavit means the merchant appraised the goods.

Here merces means goods or wares, and it is accusative plural.

That makes it the direct object of aestimavit:

• institor = subject
• aestimavit = verb
• merces = the thing he appraised

A learner may notice that merces can also mean wages in other contexts. That is true, but here the context clearly points to goods/wares.

Aestimavit is the 3rd person singular perfect active indicative of aestimare.

It means:

• he estimated
• he valued
• he appraised

So it tells us that the merchant completed the action of assessing the value of the goods.

Because paciscor is a deponent verb.

A deponent verb:

• has passive-looking forms
• but an active meaning

So pacti sunt looks like a passive perfect, but it actually means:

• they agreed
• they made a bargain
• they came to terms

It is the 3rd person plural perfect indicative of paciscor.

The subject is understood, not stated explicitly in this sentence.

Latin often leaves out subject pronouns or nouns when the verb ending already tells you the number and person. Here pacti sunt is 3rd person plural, so we know it means they agreed.

From context, the they would be the two parties involved in the agreement, probably the merchant and another person or group.

Because the subject of the second verb is not just the merchant.

In the first clause:

• institor ... aestimavit = the merchant appraised

In the second clause:

• pacti sunt = they agreed

So the subject changes from one person to more than one person. Latin does not need to spell that out if the context already makes it clear.

Also, pacti is masculine plural. With a mixed group or an unspecified group, Latin normally uses the masculine plural form.

De naulo means about the freight charge, about the transport fee, or concerning the fare.

• de = about / concerning
• naulo = ablative singular of naulum

The verb paciscor is often used with de + ablative when the thing being negotiated is expressed as the topic of agreement.

So instead of saying something like they agreed the freight charge directly, Latin says:

• de naulo pacti sunt = they agreed about the freight charge

Naulum is a noun meaning freight money, passage money, or transport fee.

It is a practical commercial word, especially useful in contexts involving shipping or transport.

Here:

• dictionary form: naulum
• in the sentence: naulo
• case: ablative singular, because it follows de

The word order is natural, but Latin word order is more flexible than English word order.

This sentence is arranged like this:

• Postquam = sets up the time clause
• institor merces aestimavit = gives the first completed action
• de naulo pacti sunt = gives the next action

A very literal order would be:

• After the merchant appraised the goods, they agreed about the freight charge.

Latin uses endings, not just position, to show who is doing what. So even if the words were rearranged, the grammar would still mostly be clear.

Usually postquam means after, and that is the most straightforward translation here.

Sometimes in flowing English, a translator may choose once or something similar:

• After the merchant appraised the goods, they agreed on the freight charge.
• Once the merchant had appraised the goods, they agreed on the freight charge.

But the core idea is definitely that the appraisal happened first, and the agreement came afterward.

Yes. Since pacti sunt is the perfect of a deponent verb, the participle pacti agrees with the subject in number and gender.

So:

• pacti sunt = masculine plural, they agreed
• if the subject were all feminine, it would be pactae sunt

This suggests either:

• the group includes at least one male, or
• Latin is using the masculine plural as the default form for a mixed or unspecified group

A learner should especially notice these:

1. Postquam + perfect indicative

   • postquam ... aestimavit
   • a very common Latin way to express after ... had done

2. Accusative direct object

   • merces is what the merchant appraised

3. Deponent verb

   • pacti sunt looks passive but means they agreed

4. Implied subject

   • Latin does not state they because the verb already shows it

5. Preposition + ablative

   • de naulo = about the freight charge

So it is a good sentence for seeing how Latin handles time sequence, business vocabulary, and deponent verbs all at once.