The modern Japanese copula だ stems from にてあり, where に (infinitive of copular なり) combined with て (conjunctive form of perfective つ) and あり (existential verb).

The て served as a connector between に and あり, much like modern Japanese て-forms connect clauses. にて essentially meant “being in the state of” before grammaticalizing.

First, there was にてあり . にて eventually contracted to で which gave way to であり which further contracted to dea -> dya -> da.

にあり: The Origin of the Early Copula なり

While にてあり is the origin of だ, we can see that なり itself is derived from case particle に + あり and merging into なり. So it makes total sense that て was able to attach to に as its the infinitive (conclusive or 連用形) form – or otherwise known as modern day stem form. にてあり therefor is nothing more than にありてあり which might look like an unusual construction from a modern day point of view but あり was also used to exhibit past tense in Early Japanese which led to the birth of た. I’ve covered the development of た in this post.

True Adjectives

The modern adjective ending い developed due to the conclusive/adnominal merger from Old and Middle Japanese’s conclusive (終止形): si and adnominal (連体形): ki into modern day い which functions both as conclusive and adnominal.

The fact that い adjectives are basically a subclass of verbs and have predicative function explains why they do not need a copula to function. This function is taken care of by conclusive い, former き.