Ｙｏｕ　ｗａｎｔ　ｔｏ　ｓａｙ　ｋａｗａｉｉ？　Ｍ ａｙｂｅ， ｋａｗａｉｉ　ｎｅ．　Ｏｒ　ｆｏｒ　ｙｏｕ，　ｋａ ｗａｉｉ　ｄａｙｏ　Ｏ．Ｋ．　Ｂｕｔ　ａｌｌ　ｉｓ 　ｕｎｄｅｒｓｔａｎｄ　ａｎｄ　ｍａｋｅ　ｓｅｎｓ ｅ．

So the phrases
この時計は高いだ　and この時計は高いだか
are both grammatically correct? It means when they say I adjective won't go with da and da is not used with ka for questions is just a matter of form and not grammar?

No.

この時計は高いだ and

この時計は高いだか

is never said. Never use だ with adjectives that end in ~い

Yonsu-san, Chryuop-san,

Sorry, my checking was too loose. "KAWAII DA" or "TAKAI DA" would not be used usually.

However, I also will not use "TAKAI DESU" or "KAWAII DESU" because "TAKAI" and "KAWAII" are to be finish form or followed by noun. I hesitate to use "KAWAII DESU" or "TAKAI DESU" because "DESU" sounds unnaturally added to already completed sentense to give polite form in these cases.

On the other hand, it sound natural to say "KAWAII DESU NE" or "TAKAI DESU NE". hmmm, it is dificult to find generarl rules.

"NE" is used to ask listener (somtime including talker himself) to agree. "NE!" alone often used as "DO you agree?". (Sorry, I did not check all cases)

Thank you for your pointing out.

RK

かわいいだ　or 高いだ would never be said or used. It is incorrect. だ never follows a ~い adjective.

かわいいです or 高いです are perfectly normal, everyday sentences. Just like in English, adjectives can come before nouns [　それは高い車です。] or stand alone.　[ その車は高いです。]

かわいいですね。 and 高いですね are also good sentences looking for agreement from the listener. The speaker assumes the listener knows the information already.

Thank you for the clarification, MMM-san and RadioKid-san! <33 I am indebted to all of you.