It appears that nai conjugates onto the renyoukei (.ku) form of i-adjectives instead of the mizenkei (.karo) form, as it does with verbs. Is this correct? Every source I've found shows the .kunai result without ever explaining what form is used.

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It appears that 「ない」 conjugates onto the 連用形{れんようけい} (~~く) form of i-adjectives instead of the 未然形{みぜんけい} (~~かろ) form, as it does with verbs. Is this correct?

Yes, it is correct. As a Japanese-learner, I suppose you would just have to memorize the rule in forming the negative forms. Keep making mistakes and eventually, only the correct forms will sound good in your own ear.

Why does this difference exist, then?

In short, because we are, strictly speaking, talking about two different 「ない's」 here -- two different words with the same spelling, that is, at least historically.

「ない」 attached to verbs is an auxiliary verb while 「ない」 attached to adjectives is an adjective.

Tips on how to tell the two 「ない's」 apart:

Try replacing the 「ない」 with 「ぬ」 in phrases containing 「ない」. If it still makes sense, it is the auxiliary verb 「ない」.

「かわいくない」⇒「かわいく」 Makes no sense; Must be the adjective 「ない」.

「話{はな}さない」⇒「話さ」 Makes perfect sense; Must be the auxiliary verb 「ない」.

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