Knave said:
Aw man!!!
You're serious?! There is no real rule for determining which adjective falls under which category?
There is a rule of sorts. First of all, ignore romaji (that is, just because you see "i" at the end doesn't mean that it ends in い). Second, if it ends in い it is an -i adjective unless: 1) the final い is part of a kanji reading, 2) the final い forms the infinitive (pre-masu) form of a verb. Here are some examples:
きれい - な adjective (or 形容動詞) because the れい is actually the reading of the second kanji: 麗. This word in kanji is 綺麗.
大きい、小さい、青い、鋭い、らしい、珍しい、おかしい - all -i adjectives (or 形容詞). As you can see, all have a final and independent い.
嫌い - な adjective (形容動詞), because it comes from the verb 嫌う, meaning dislike.
In my experience any time you see
-ei at the end of a word, it isn't an -i adjective, because it's usually there as the vowel lengthener. The only word that I could think of that ends in い and is a -na adjective is the one that Elizabeth already mentioned:
kirai, and I explained that one. So, if it ends in
-ai, -oi, -ui, or
-ii, it's a safe bet that it's an -i adjective.