美しいだ is wrong. 美しいです is vulgar.

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May 26, 2011 19:36 Japanese_language adjective 形容詞 「です」
As you know, i-adjectives are not followed by だ. I said once that 美しいです also sounds to me a bit strange, though it is considered correct by orthodox grammar. However, I found out a book that supports my feeling. The book, by Kindaichi Haruhiko, wrote that i-adjective + です was regarded vulgar before the war. I guess that many things changed when Japan lost the war and 美しいです was probably also one of them.

I feel that when using i-adjective + です, there is a gap between one who has felt something and one who talks about it.

Cf.
金田一春彦『日本語 新版(下)』134頁(岩波新書新赤版3,岩波書店,1988)

See also:
http://lang-8.com/satoshi/journals/945797/ (Is 私はほんとに忙しいでした。 wrong?)
http://lang-8.com/satoshi/journals/920039/ (I can’t understand why many learners write 難しいだ。)