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I am complete beginner and I've got a question that I wish to ask you: When negating an i-adjective (-くない), should we, as in the affirmative form, omit the declarative "だ"? For example, is it "あのゲームは楽しくないだよ!" or "あのゲームは楽しくないよ!".


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Since you are a complete beginner, this is probably not relevant to you, but to anyone else who may come across this question, it is worth mentioning that there are some dialects which allow だ after i-adjectives, such as 遠州弁 or 三河弁. –  Darius Jahandarie Mar 14 '14 at 0:38

2 Answers 2

You cannot attach a 「だ」 directly to an i-adjective, either in its affirmative or negative form. (But believe me, many Japanese-learners make this mistake.)

Incorrect: 「[楽]{たの}しいだ」、「楽しくないだ」、「[大]{おお}きいだ」、「大きくないだ」, etc.

Attaching a よ or ね is correct and very natural --- affirmative or negative.

Correct: 「楽しいね」、「大きくないよ」、「かわいいね」、「おいしくないよ」, etc.

If you absolutely MUST use a だ for some reason, you can do so by inserting a 「の」 or informally, a 「ん」 between the i-adjective and the 「だ」 --- affirmative or negative.

Correct: 「楽しいのだ」、「[小]{ちい}さいんだ」、「[高]{たか}いんだ」、「[赤]{あか}いのだ」、「かわいくないのだ」、「大きくないんだ」, etc.

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I do know that we musn't use "da" with an i-adjective when it is in its affirmative form, but I was wondering about the negative form. So thanks for your answer, it sure clears things up. –  Nun Mar 14 '14 at 0:01

When negating an i-adjective, you change い for くない. The ない part is an auxiliary i-adjective, so you should omit だ, just as you do for i-adjectives.

A bullet train is fast.

A broken bullet train is not fast.

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I would personally say だ was never there in the first place, rather than say that it's omitted. –  snailboat Mar 13 '14 at 23:58
Sure, but the question body said "should I omit it?", so I chose to use the same language. –  Earthliŋ Mar 14 '14 at 0:01

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