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Why is it valid to say おいしそうだ but not おいしいようだ? Not in terms of grammar, but in
terms of what is natural and what's not.

Why do I mostly see i-adjectives taking そう but not よう or みたい? Is there some
logic behind it? Some difference in nuance?
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nuances
Why is it valid to say おいしそうだ but not おいしいようだ?
As you may be aware, both are grammatical.

From 大辞泉, the relevant meanings of そうだ and ようだ are:

>
> [そうだ](https://dictionary.goo.ne.jp/word/%E3%81%9D%E3%81%86%E3%81%A0/#jn-129024)
> 動詞・助動詞などの連用形、形容詞・形容動詞などの語幹に付き、語幹が1音節の形容詞には「さそうだ」、また助動詞「たい」「ない」に付くときは「たそうだ」「なそうだ」の形をとる。
> **様態** の意を表す。…というようすだ。今にも…するようなようすだ。
>
> [ようだ](https://dictionary.goo.ne.jp/word/%E6%A7%98%E3%81%A0/#jn-226613)
> （主に文末に用いて）不確かな、または **婉曲** (えんきょく) な断定の意を表す。「この機械はどこも故障していないようだ」
>
> (emphasis added)

Both may be translated to 'to seem' in English, but そうだ is about
様態=appearance, how it **looks** and ようだ makes a sentence **indirect**.

You see おいしそうだ more often mostly because that's what people say 'it looks
tasty' more often.
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2022-11-15T23:00:15.123
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If you are curiously seeing something and saying something subjectively based
on your direct visual observation, you must use そうだ. If ようだ was used even
though you're directly seeing something, it would sound like you're
indifferent or trying to put aside your direct observation.

  * **おいしそうだ** : Looks delicious (to my eyes)!
  * **おいしいようだ / おいしいみたいだ** : (Putting aside how it actually appears to me, according to a guidebook, rumor, etc) it looks that this is delicious. / (I'm not particlarly interested, but since people say it's good,) this is probably delicious.
  * **面白そうだ** : Looks interesting! / Sounds fun!
  * **面白いようだ** : (According to what I've heard) looks like this is interesting (though I'm not saying I'm interested).
  * **嬉しそうだ** : She (evidently) looks happy.
  * **嬉しいようだ** : (My eyes cannot quite see it, but) this seems to mean she is happy.  
(Imagine you gave a gift to a tsundere character.)

So while both are perfectly valid and grammatical, おいしいようだ is far less common
in practice.

See my answer here, too: [Difference between そうです、 ようです and
らしいです.](https://japanese.stackexchange.com/q/16312/5010)
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