This article is from the Japanese FAQ, by TANAKA Tomoyuki tanaka@cs.indiana.edu and Olaf Meeuwissen olaf@IMSL.shinshu-u.ac.jp with numerous contributions by others.
>
> `desu' after an adjective has
> no other purpose than to provide politeness,
(ungrammatical phrases/utterances are marked with asterisks.)
"*akai desu" is ungrammatical and sounds juvenile (infantile,
baby-talk, as "akai desyu").
it sounds like a Japanese phrase spoken by a gaijin.
the correct way to make the assertion "akai" into a more polite
form is "akou gozaimasu" (which, i admit, sounds anachronistic
and over-polite).
since DESU and DA are grammatically equivalent, allowing
*AKAI DESU would result in allowing *AKAI DA.
strange: *abunai desu/da
correct:
kiken desu/da
abunou gozaimasu
abunai no desu/da
abunai n desu/da
i think this is a position shared by older (purist) writers
and linguists.
"ookii desune" sounds acceptable.
 
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