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


2 Answers 2


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.

  • 9
    @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. Feb 6, 2017 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, 2017 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. Feb 7, 2017 at 17:23
  • This explanation is an excellently out of the box way of thinking about i adjectives. Thank you!
    – Eginma
    Feb 8, 2017 at 0:35

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.