In "Captivi" Act 3, Scene 4 (Plautus), between lines 40-45 there is the following dialogue:
HEGIO: plus quidem quam tibi aut mihi nam ille quidem, quem tu hunc memoras esse, hodie hinc abiit Alidem ad patrem huius.
ARISTOPHANTES: quem patrem qui servos est?
TYNDARUS: et tu quidem servos es,
The translation (Perseus Pl. Capt. 3.4):
HEGIO: More indeed than either you or myself for he, in fact, who you say that he is [pointing to Tyndarus] has set out hence today for Elis, to this person's father.
ARISTOPHANTES: What father, when he's a slave?!
TYNDARUS: And so are you a slave,
Context: In Roman society slaves were not considered to have any legal existence; and, therefore to have neither parents nor relations. Hence the shock-horror reaction of Aristophantes.
In "quem patrem qui servos est" why is a sentence beginning with an accusative? Is the reader to understand that Aristophantes has repeated a word ("patrem") from Hegio, with an embellishment ("quem") in a mocking way? Alternatively, is "ad" to be understood, "ad quem patrem..." = "To what father..."?
The expected line would be "quis (adjectival form = "what") pater, qui servus est".
Why, in the relative clause, "qui servos est", is "slave" given as accusative plural?
Firstly, there is only one person under discussion ("pater"/ "patrem"); secondly, in this relative clause "when he's a slave"/ ("who is a slave") is a state of affairs, requiring a nominative ("servus"); not the action of a verb on a noun, requiring an accusative ("servum"/ "servos").
To add to the confusion, Tyndarus replied to Aristophantes with, "et tu quidem servos es", using the accusative plural "servos".
What is going on, here?