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


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.

  • 8
    @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

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
  • 2
    @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

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.

  • 2
    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

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.