Mercatrix pretium trium persicorum duos denarios poscit.

Mercatrix is in the nominative singular, which is the case normally used for the subject of a sentence. It also matches the verb poscit, which is third-person singular: the merchant asks.

Also, mercatrix is a feminine noun meaning female merchant, female trader, or saleswoman.

Poscit is:

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

It comes from poscō, poscere, meaning to ask for, to demand, or in a commercial context to ask as a price.

So mercatrix ... poscit means the merchant asks/demands.

Because pretium is the direct object of poscit.

Latin is treating price as the thing being asked for:

• pretium poscit = she asks a price

So pretium is accusative singular because it is what the subject is asking.

Because duos denarios explains what the pretium is: a price, namely two denarii.

A helpful way to see it is:

• pretium ... duos denarios = the price ... two denarii
• or more naturally, a price of two denarii

So duos denarios is in apposition to pretium, and appositional words usually stand in the same case.

Because trium is the genitive plural form of tres.

The noun persicorum is genitive plural, so the number modifying it must also be genitive plural:

• nominative/accusative: tres
• genitive: trium
• dative/ablative: tribus

So:

• tres persica = three peaches
• trium persicorum = of three peaches

Because after pretium, Latin often puts the thing priced in the genitive.

So:

• pretium persicorum = the price of the peaches
• pretium trium persicorum = the price of three peaches

English often uses of or sometimes for here, but Latin uses the genitive case instead.

It is often a bit stronger than a simple asks. Poscō can mean:

• ask for
• demand
• require

In a buying/selling context, it can mean something like:

• asks as a price
• charges
• demands

So in this sentence, asks is fine, but demands or charges may sometimes fit the tone better depending on context.

Latin word order is much more flexible than English word order because the case endings show each word’s role.

Here the sentence is:

• Mercatrix = subject
• pretium trium persicorum duos denarios = the price phrase
• poscit = verb

Latin often puts the verb near the end, so this order is very natural.

You could rearrange the sentence in other ways and still keep the same basic meaning, because the endings still show the grammar.

Latin does not have articles like English the and a/an.

So mercatrix can mean:

• a merchant
• the merchant

and context tells you which is better in translation.

The same is true for the other nouns in the sentence.

Yes. Mercatrix is the feminine form, and mercator is the usual masculine form.

So:

• mercator = male merchant / merchant
• mercatrix = female merchant

Latin often has paired words like this for male and female people.