Postquam nautae navem exonerant, institor merces aestimat.

Postquam introduces a time clause. It tells you that the action in its clause happens first, and the action in the main clause happens afterward.

So the sentence is divided like this:

Postquam nautae navem exonerant
institor merces aestimat

The first part gives the earlier action, and the second part gives what happens next.

Nautae is nominative plural, so it is the subject of exonerant.

A learner may pause because -ae is a very noticeable ending, but here it is the normal nominative plural ending of a first-declension noun. So nautae means the sailors as the doers of the action.

Also, exonerant is third person plural, and that matches nautae perfectly.

Yes. Nauta, nautae is a first-declension noun, but it is masculine in meaning and gender.

That is not unusual in Latin. A number of nouns for jobs or roles traditionally done by men are first declension but masculine, such as poeta and agricola.

So:

• nauta = sailor
• nautae = sailors

Even though the endings look like many feminine nouns, the word itself is masculine.

Because it is accusative singular, the direct object of exonerant.

The dictionary form is navis, meaning ship. That is a third-declension noun, and its accusative singular is navem.

So:

• navis = ship, in the nominative singular
• navem = ship, as the direct object

In this sentence, the sailors are unloading the ship, so Latin puts navis into the accusative: navem.

Exonerant is:

• present tense
• indicative mood
• active voice
• third person plural

It comes from exonerare, meaning to unload or to relieve of a load.

The -ant ending tells you it is third person plural, so it means they unload or they are unloading, depending on context.

Because the subject is nautae, the form makes sense: the sailors unload.

Because institor is nominative singular, and it matches the singular verb aestimat.

So in the second clause:

• institor = the trader / merchant, as subject
• aestimat = he assesses / values

A native English speaker may expect the subject to come first very rigidly, but Latin relies more on case endings and verb endings than on word order. Here, the grammar tells you that institor is the doer of the action.

Because merces here is accusative plural of merx, mercis, meaning goods or wares.

For this noun, the nominative plural and accusative plural are both merces. That means the form by itself could be either subject or object. You figure out its role from the rest of the sentence.

Here, institor is clearly the subject because it is nominative singular and matches aestimat. So merces must be the direct object: the merchant assesses the goods.

Aestimat is:

• present tense
• indicative mood
• active voice
• third person singular

It comes from aestimare, meaning to estimate, assess, value.

The -t ending tells you it is third person singular: he, she, or it assesses. Since the subject is institor, the sense is the merchant assesses the goods.

In a simple teaching sentence like this, the present tense often expresses a regular or typical sequence:

After the sailors unload the ship, the merchant assesses the goods.

So the idea is not necessarily one specific past event. It can mean something like whenever this happens, this is what follows.

If Latin were narrating one completed event in the past, you would often see past tenses instead. But here the present makes good sense as a general sequence.

Because that is a very common Latin word order.

Latin word order is flexible, since case endings already show what each noun is doing. Even so, Latin often places the verb at the end, especially in straightforward prose and textbook sentences.

So both clauses follow a very typical pattern:

• nautae navem exonerant
• institor merces aestimat

English depends much more heavily on word order. Latin can move words around more freely without losing clarity.

Because Latin has no articles.

English distinguishes between a ship and the ship, but Latin normally does not use separate words for that. A noun like navem can mean a ship or the ship, depending on context.

The same is true for institor and merces. Context and the overall situation tell you whether English should use a, an, the, or sometimes no article at all.