The king's wife undressed?

Χαίρετε!

I am supposed to write, “The king’s wife undressed” in Greek.

Dr. Mastronarde’s Introduction to Attic Greek, Unit 24, page 202, says that undress is a “middle/passive, 2nd aorist.”

The answer book wrote, “ἡ τοῦ Βασιλέως γυνὴ ἀπέδυ.”

The ending of the verb is confusing me. It looks like a secondary active ending. Shouldn’t it be a secondary
middle ending? Shouldn’t there be a “το” at the end?

Right, but as LSJ states for ἀποδύω, the second aorist active ἀπέδυν is intransitive. Basically, it works as if it were middle,

Εὐχαριτῶ!