Learn Russian: Здесь продают хорошую одежду.

It is the accusative (direct object) form of the feminine adjective хорошая (good). Since it describes the feminine noun одежду, it must match its case. In the accusative case, feminine adjectives ending in -ая change to -ую.

The English translation says "clothes," which is plural. Why is одежду in the singular form?

In Russian, the word for clothing or clothes, одежда, is an uncountable singular noun, much like the word "clothing" in English. Therefore, we use the singular feminine accusative form here instead of a plural.

Who is the subject of продают? Why isn't there a pronoun like они (they)?

When Russians make a general statement about what happens at a place (like a store), they often use the "they" form of the verb without the pronoun они. This is a common way to say "clothes are sold here" or "one sells clothes here."

The dictionary form of the verb is продавать. Where did the -ва- go in продают?

Verbs ending in -авать (like продавать, to sell, or давать, to give) drop the -ва- when conjugated in the present tense. So the base becomes прода-, and we just add the standard "they" ending -ют to get продают.