In classical Japanese, many uses of 形容詞{けいようし} had あり "embedded" in them, e.g.:
- 熱からず = 「熱し」の連用形+「あり」の未然形{みぜんけい}+「ず」
- 熱かりたり = 「熱し」の連用形+「あり」の連用形{れんようけい}+「たり」
- 熱かれ = 「熱し」の連用形+「あり」の命令形{めいれいけい}
- 熱かる人 = 「熱し」の連用形+「あり」の連体形{れんたいけい}
while others did not:
- 熱くす = 「熱し」の連用形+「す」
- 熱し = 「熱し」の終止形{しゅうしけい}
- 熱き人 = 「熱し」の連体形{れんたいけい}
It seems that there is a plain form and an あり form for most of the conjugations:
plain あり 未然形 -ku -kara 連用形 -ku -kari 終止形 -si missing 連体形 -ki -karu 已然形 -kere -kare 命令形 missing -kare
My question is why certain usages of 形容詞 used the あり version and others didn't. Did the あり have some sort of semantic meaning?
(The original motivation for this was the question of why the past tense of 形容詞 is かった rather than くた or った or いた, while the て-form is くて.)
(P.S., the majority of my experience with classical Japanese is reading about its grammar, not actually reading old poems or stories. I suspect if I had more actual experience I'd be able to understand why あり is required in these places.)