grammar - Why can't だ be used after an I-adjective?

I have been constantly told that だ can not be placed after an I-adjective. Why is that?

Answer

だ is the plain-form copula (the "is; to be" word). In the plain form, い adjectives already form a complete predicate (the piece of a sentence or clause that can complete that sentence or clause). In translation, it's like the い adjective already includes the "is" meaning -- so 速い would be "[it] is fast", not just "fast".

Since だ is only used to provide a way of finalizing plain-form clauses, it's redundant in cases where the plain-form clause is already a complete predicate -- such as when it ends in an い adjective. Saying 速いだ might be like saying "[it] is is fast" in English -- the meaning is clear, but it's grammatically wrong and it sounds wrong.