Learn Farsi: دیوارها تمیز است.

You simply add -hâ to the end of the singular noun divâr (wall) to make it divâr-hâ (walls). It is the standard, regular way to make a word plural in Farsi and can be attached to almost any noun.

Wait, why is the verb ast (is) singular when "walls" is plural? Shouldn't it be "are"?

This is a very important and surprising rule in Farsi! When a plural subject is an inanimate object (non-living things like walls, books, or cars), the verb is almost always singular. So, Farsi literally says "The walls is clean." If the subject were living things (like people), you would use a plural verb.

Does the adjective tamiz (clean) need a plural marker too, since it describes multiple walls?

No, it doesn't. In Farsi, adjectives never change their form to match the noun they are describing. Whether it is one clean wall or a hundred clean walls, the adjective stays exactly the same: tamiz.

Why is the verb ast (is) at the very end of the sentence?

Farsi places the verb at the very end of the sentence. In a simple descriptive sentence like this, the pattern is Subject (divâr-hâ) + Adjective/Description (tamiz) + Verb (ast).