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I have been constantly told that だ can not be placed after an I-adjective. Why is that?

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だ is the plain-form copula (the "is; to be" word). In the plain form, い adjectives already form a complete predicate (the piece of a sentence or clause that can complete that sentence or clause). In translation, it's like the い adjective already includes the "is" meaning -- so 速い would be "[it] is fast", not just "fast".

Since だ is only used to provide a way of finalizing plain-form clauses, it's redundant in cases where the plain-form clause is already a complete predicate -- such as when it ends in an い adjective. Saying 速いだ might be like saying "[it] is is fast" in English -- the meaning is clear, but it's grammatically wrong and it sounds wrong.

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    @CraigHicks, です is a different case -- see the link posted above by naruto, japanese.stackexchange.com/a/35951/5010. In summary, plain form だ only provides closure of a predicate, while polite form です also provides social register information (in this case, politeness) -- so い adjective + です is correct, and is in fact the required form for polite-register い adjectives. – Eiríkr Útlendi Feb 6 '17 at 19:42
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Grammatically speaking, there really are no adjectives in Japanese. i-adjectives are just special verbs. i-adjectives have many of the same inflections as do verbs, and they fulfill a grammatical role essentially equivalent to that of verbs. Therefore, 電車は速い is a complete sentence meaning "the train is fast", where 速い is the predicate.

i-adjective + です could be considered a special construction for the sake of politeness, similar to verb + ます. However, attaching an adjective to だ or でした, etc., would be ungrammatical or at least unidiomatic (because you would have two predicates). If you need the polite form of the past tense, instead of 早いでした, you would say 早かったです.

I know this is really confusing for many people. I used to be confused about the same question as well, but when I realized that i-adjectives are really a special category of verbs, it is easy to understand what is going on.

  • Wow! I have never consider i-adjective as a kind of verb before. Thanks – Anh Tuan Feb 7 '17 at 3:27
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    @AnhTuan, many of the conjugated forms of い adjectives are contractions with ある, where the contraction has so normalized that folks have forgotten about it. Examples: 長かった = 長く + あった, 長かれ = 長く + あれ, etc. – Eiríkr Útlendi Feb 7 '17 at 17:23
  • This explanation is an excellently out of the box way of thinking about i adjectives. Thank you! – Eginma Feb 8 '17 at 0:35
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well, that is kind of truth for くん読みwords; there is really a verb 早う, rarely used today. And in the past, much more いadjectives had う,る, or ふ form. In contrast, な adjectives are 外来語 and follow Chinese contextual rules.

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    早う is not a verb -- it is an adverb, a variation on standard 早く used in Kansai. It's the same adverb found in the common expression for "good morning" -- お[早]{●}[う]{●}ございます. The shift from hayaku to hayau is the result of a regular sound change in the Ashikaga or Muromachi periods, called ウ音便 (u onbin). More in Japanese at the Japanese Wikipedia. This loss of the medial /k/ also happened in the adjective form, producing the modern hayai from older hayaki. – Eiríkr Útlendi Apr 26 '18 at 6:25
  • From the other side, In Kansai, there are a lot of pre-reform words and expressions used. And that brings us back to xuq01's post above. Please, read it carefully before checking the dictionary. – Asdf Apr 28 '18 at 9:25

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