Vir mercatorem accusat, sed mercator se defendit.

Because vir is the subject (the one doing the accusing), so it’s nominative. Mercatorem is the direct object (the one being accused), so it’s accusative. Latin marks grammatical roles mainly by case endings rather than by word order.

You mainly rely on case endings:

• vir (nominative singular) = the accuser
• mercatorem (accusative singular) = the accused
  Even if you rearranged the words, those endings would still show the roles (e.g., mercatorem vir accusat still means The man accuses the merchant).

sed means but. It’s a coordinating conjunction that links two independent clauses. It doesn’t change the case or verb forms; it just signals contrast between the clauses:

• Clause 1: Vir mercatorem accusat
• Clause 2: mercator se defendit

Because the subject changes. In the second clause, mercator is nominative and becomes the new subject: the merchant defends himself. Latin often repeats or switches subjects explicitly like this for clarity.

Because in the second clause the merchant is doing the action (defendit), so he must be in the nominative: mercator.
In the first clause he receives the action (accusat someone), so he’s the direct object and is accusative: mercatorem.

se is the reflexive pronoun meaning himself / herself / itself / themselves, referring back to the subject of the same clause. Here it refers back to mercator, so it means the merchant defends himself.
eum would mean him (a non-reflexive object), typically referring to some other male person, not the subject.

No. se (accusative/ablative) doesn’t show gender, and it’s used for both singular and plural subjects. The meaning depends on the subject:

• mercator se defendit = the merchant defends himself
• mercatores se defendunt = the merchants defend themselves

• accusat comes from accūsō, accūsāre, accūsāvī, accūsātum (1st conjugation), meaning accuse
• defendit comes from dēfendō, dēfendere, dēfendī, dēfēnsum (3rd conjugation), meaning defend

Both are present tense, 3rd person singular, active indicative:

• accusat = he accuses
• defendit = he defends
  You recognize present tense here by the endings:
• 1st conjugation present 3rd sg: -at → accusat
• 3rd conjugation present 3rd sg: -it → defendit

Latin typically doesn’t need subject pronouns because the verb ending already indicates person and number. accusat and defendit already mean he/she/it accuses/defends. A pronoun like is could be added for emphasis or clarity, but it isn’t required.

Not as written. mercator se defendit specifically means the merchant defends himself because se must refer back to the subject mercator.
To mean defends him (someone else), you’d expect something like eum defendit (assuming him refers to the man or another male).

Latin has no articles (a/an/the), so mercatorem could be either a merchant or the merchant depending on context. Many translations use the if the merchant is already known in the story, but the Latin form itself doesn’t force that.

Classical Latin manuscripts didn’t use punctuation the way modern English does. In modern printed Latin, a comma before sed is common and helpful, especially between two full clauses, but punctuation is more about readability than strict grammar.