だ versus な in causal subordinate clauses Why do causal subordinate clauses ending in - take as a means to connect nouns and adjectives (e.g. ), whereas causal subordinate clauses ending in - take (e.g. )? I read that derives from +, whereas derives from +, but why would one causal conjunction require , whereas another one requires ? Just trying to make sense of it all… Thank you!

This explanation is tautological but I just have to say it's because follows a terminal form, which of the copula is while is a kind of noun, which needs an attributive form to be modified, which of the copula is .

Their etymology has nothing to do with this issue.