質問: 「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.)
回答: I have come to the following conclusion: **The あり form of 形容詞 arose to support 助動詞.** The non-あり form is only used with 名詞 (via the 連体形), 動詞・形容詞 (via the 連用形), and 接続助詞. To make sure that this explanation is not circular, 助動詞 and 接続助詞 need to be differentiated using some other property — thankfully, 接続助詞 cannot be predicative while 助動詞 can. Some examples... > > 形容詞+助動詞 > > > * 未然形:熱からず、熱からむ > * 連用形:熱かりき、熱かりけり、熱かりたり > * 連体形:熱かるべし > > > 形容詞+動詞 > > > * 連用形:熱くなり、熱くす > > > 形容詞+接続助詞 > > > * 未然形:熱くば > * 連用形:熱くて > * 已然形:熱ければ、熱けれど、熱けれども > > > (In the case of 接続助詞「て」, it's true that it comes from 助動詞「つ」, but it seems that it was reanalyzed as a 接続助詞, allowing for the wider distribution: ○熱くて、x熱くつ.) I think the reasoning goes like this: 助動詞 need to attach to a verb (whether this is a purely syntactic constraint, or some sort of semantic one, I don't know), so あり was inserted to help out. 接続助詞 on the other hand have no such constraint. And then, once the あり form arose, some things that could already be done gained a new form, such as 熱からば and 熱かる人 — I do not think their meaning was different from the non-あり form.