Learn Farsi: کلاس‌های اختیاری از این ممنوعیت معاف نیستند. - Optional classes are not exempt from this prohibition.

be esthesnâ-ye is a preposition meaning except for, while mo'âf is an adjective meaning exempt. To use the lesson's phrase, you would have to rewrite the sentence entirely, for example: All classes, be esthesnâ-ye optional classes, are banned. Notice that mo'âf always pairs with the preposition az (from) in this context.

How is the noun mamnu'iyyat (prohibition) built?

It is built from the adjective mamnu' (forbidden). The suffix -iyyat is used to create formal abstract nouns, much like the English suffixes -ity or -tion. So, mamnu' becomes mamnu'iyyat.

What is the root of the word ekhtiyâri (optional)?

The root is the noun ekhtiyâr, which means choice, authority, or free will. Adding the adjective-forming suffix -i creates ekhtiyâri, meaning something is left to your own choice.

Since kelâs-hâ (classes) is an inanimate plural subject, why is the plural verb nistand used instead of the singular nist?

While inanimate plural subjects often take singular verbs in spoken or casual Farsi, formal written Farsi frequently uses plural verbs for them. Given the formal vocabulary here (mamnu'iyyat, mo'âf), the plural verb nistand perfectly matches the official tone.