338. In the perfect middle system the final consonant in the stem caused certain changes (paradigms, § 339).
339. Perfect Middle System
a. The endings -νται, -ντο after a consonant being almost unpronounceable, compound forms are used.
b. In the subjunctive and optative compound forms are used.
c. Forms in -νσαι and -νσο do not occur; compound forms may have been used.