Discipula aperte mercatorem accusat, quia pretium nimium petivit.

Discipula is nominative singular because it is the subject of the main verb accusat. In Latin, the subject is typically put in the nominative case: The (female) student accuses…

Mercatorem is accusative singular because it is the direct object of accusat. The verb accusare (to accuse) takes the person accused in the accusative: accusat mercatorem = she accuses the merchant.

Aperte is an adverb meaning openly / plainly. It modifies accusat (how she accuses). Adverbs in Latin are flexible in placement, so you might also see:

• Discipula mercatorem aperte accusat
• Aperte discipula mercatorem accusat The meaning stays basically the same, though emphasis can shift slightly.

Latin word order is flexible, but a very common neutral pattern is SOV (subject–object–verb). Here:

• Discipula (S)
• mercatorem (O)
• accusat (V) This is typical, especially in straightforward prose.

Quia means because and introduces a causal subordinate clause. Everything after quia explains the reason for the accusation:

• quia pretium nimium petivit = because he demanded too high a price.

The subject of petivit is understood to be mercator (the merchant). Latin often omits pronouns and repeats of obvious nouns, because the verb ending already signals a person/number, and context supplies who it is. So:

• petivit = he demanded (i.e., the merchant).

Petivit is perfect active indicative, 3rd person singular, from petere (to seek / demand / ask for). The perfect often translates as:

• he demanded
• he did demand
• sometimes he has demanded (depending on context)

Pretium is accusative singular because it is the direct object of petivit: he demanded a price. In English we often say he asked for a price, but Latin can use a direct object with petere.

Here nimium is functioning as an adverb meaning too much / excessively. It modifies petivit (the demanding was excessive), and the idea is that the merchant demanded too high a price.

You can also meet nimius, -a, -um as an adjective meaning excessive, but in this sentence nimium is best taken adverbially.

Yes. Latin can express this idea in multiple ways, for example:

• quia pretium nimis magnum petivit = because he demanded a too large price
• quia pretium nimis altum posuit = because he set a too high price (different verb) Your sentence uses nimium neatly to convey too much without adding an extra adjective.

Classical Latin manuscripts didn’t use modern punctuation consistently. The comma is largely an editorial/modern teaching convention to mark the boundary before a subordinate clause. It helps readability, but it’s not “original” in the way punctuation works in modern English.

Accusat comes from accusare (1st conjugation):

• accuso, accusare, accusavi, accusatum = to accuse
  Accusat is present active indicative, 3rd person singular: she accuses / is accusing.