Questions & Answers about Servae mensam in triclinio ad cenam instruunt.
Because instruunt is third person plural: they set/prepare. The form servae can, in theory, mean several things:
- nominative plural = female slaves / maidservants
- genitive singular = of the female slave
- dative singular = to/for the female slave
But only nominative plural fits naturally with the plural verb instruunt. So here servae is the subject: the female slaves / maidservants.
Mensam is accusative singular of mensa.
It is the direct object of instruunt, so it answers the question What are they setting/preparing?
Answer: the table.
This is very typical in Latin: the thing directly affected by the verb goes into the accusative.