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?




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.




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.

