Basically, there is no such thing as の-adjective. It's a noun that can be modified to adjectives in the English language. When you run into words that can take either な or の, they are called adjectival nouns. の is used when you need to label or categorize a noun, while な is used when you want to describe a noun. 田中さんは普通のサラリーマンだ。 田中さんは普通なサラリーマンだ。 Both means 'he is an ordinary salaryman', but when we use の, we’re not actually describing the quality of Tanaka-san. Instead, 普通の (adjectival noun) acts as a category the the slaryman. On the other hand, 普通な (adjective) offers a description of Tanaka-san's own personality or character (that has nothing to do with him being a salaryman). We've made a more detailed post about it, too. Hope this helps! https://cotoacademy.com/%E3%81%AA-adjectives-and-%E3%81%AE-adjectives/