Question as the title follows, Why Isn’t きれい (pretty) considered an い adjective?
きれい, while often written using just hiragana, actually comes from the kanji word 綺麗. Because of that, it is a na-adjective.
い-adjectives all end in い but not all adjectives that end in い are い-adjectives. You just have to learn the exceptions as they come along, Japanese has many exceptions.
As some have said, きれい is an example of a word that just happens to end it い but not in hiragana い. It is usually written in hiragana, but it comes from the word 綺麗 that is written in kanji. As you cannot conjugate the ending of the kanji to make past-tense or negative, etc, it has to be a na adjective. ゆうめい (famous) is another example of a na adjective that ends in an い and so one you have to memorise.
ゆうめい would be written 有名 if in kanji. So it must behave like a na-adjective and follow na-adjective rules for conjugation.
won’t this be true of any adjective whose ending い is simply an elongated え sound as well, regardless of kanji writing? that was how my first professor explained きれい being a na-adjective.
I think that’s the easiest way to remember it… Especially if you donot remember the kanjis for the word.
It’s even more fun with 大きい and 大き(な)
You can have a more in-depth read here: i-adjectives-used-as-na-adjectives-is-there-a-difference
Tae Kim explains this prettt well, I think. Adjectives where there is a kanji with い after it get conjugated without changing the Kanji, just the い. Because the い sound in きれい is “trapped” in the Kanji in the root word, it can’t be conjugated easily like a true い adjective