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According to the book I am reading, "Sakura is beautiful" is written as

さくらはきれいです

and "Sakura is a beautiful flower" is

さくらはきれいなはなです

,but when using たかい the na form isn't used. For example:

ふじさんはたかいで

ふじさんはたかいやまです

But not

ふじさんはたかいなやまです

Community
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Aditya Dev
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    `when using たかい the na form isn't used.` たかい is not a na-adjective, but an i-adjective. きれい happens to end with an い, but it's not an i-adjective but a na-adjective. Check their word class in your dictionary. – Chocolate Oct 11 '18 at 14:59

2 Answers2

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It is because there are い adjectives and な adjectives.

The attributive form of a な adjective ends with な, such as in きれいなはな.

The attributive form of an い adjective ends with い, such as in たかいやま.

When they are placed before です, in the case of an い adjective, you use the plain form, such as in ふじさんはたかいです. In the case of a な adjective, you use the stem of the word, such as in さくらはきれいです.

Yuuichi Tam
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    `you use the attributive form, such as in ふじさんはたかいです` -- 「たかいです」の「たかい」は、連体形(attributive form) じゃなくて 終止形 (end form / terminal form とか) です、一応・・ – Chocolate Oct 11 '18 at 15:36
  • @Chocolate どっちにしようか迷いました。「です」の前は体言なので、連体形にしたほうがいいと思いました。動詞は連体形ですよね。形容詞の場合は、連体形も終止形も同じなので結果的には変わらないのですが。何故形容詞は終止形なのでしょうか? – Yuuichi Tam Oct 11 '18 at 15:52
  • 助動詞の「です」を辞書や文法書で調べると、「 **形容詞の終止形**に付いて・・」「形容動詞の語幹に付いて・・」「体言に付いて・・」等と書かれていますので・・　「何故形容詞は終止形？」と言われましても・・ – Chocolate Oct 11 '18 at 15:59
  • 私の古い辞書には載ってなかったので、ありがとうございます。 – Yuuichi Tam Oct 11 '18 at 16:12
  • @YuuichiTam 大前提として、日本語の形容詞は、英語と違ってbe動詞に当たるダを必要とせず、終止形という活用形で直接文章を終えることができます(例「空は青い。」)。これが日本語の形容詞は実際には動詞に近いとか言われている所以です。さて敬体(ですます体)ではダをデスに変えるというのが原則ですが、形容詞を丁寧に言う場合は、そもそも変えるべきダが存在しないため、単なるpoliteness markerとしてデスをくっつけることが認められています。そんなわけで「高いです」の「高い」は終止形です。[この回答](https://japanese.stackexchange.com/a/15304/5010)や[この回答](https://japanese.stackexchange.com/a/35951/5010)が参考になると思います。 – naruto Oct 11 '18 at 16:22
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This is unfortunately one of the parts of Japanese that you'll have to just remember. Some Adjectives can also be said with い AND な。

It happens with a few adjectives, and this occurs in daily speech as well.

In general, especially when you're first learning, I would simply use the い form for all ”い” adjectives. Realize that there are specific な　only adjectives as well (which do not take い).

If you want to delve further, this has been answered in depth here:

Adjectives functioning both as イ- and ナ-adjective

UselessGiraffe
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  • What you've said is true, but I imagine the downvotes are because it's not related to what the question is about. The question is merely a misunderstanding of the class きれい falls into. – Leebo Oct 11 '18 at 23:24
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    @Leebo おおきな or ちいさな are not na adjective. It's misleading to say "some adjectives can also be said with い and な". – user4092 Oct 12 '18 at 05:27
  • Yes you're right, it's wrong in more than one way. Though I don't think it actually said those were な adjectives, as I said it's just an unrelated area grammar. – Leebo Oct 12 '18 at 05:33