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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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2017-02-06T16:58:26.383
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2018-04-26T06:04:31.547
2017-02-06T19:52:46.450
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question
grammar word-choice i-adjectives copula register
Why can't だ be used after an I-adjective?
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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2017-02-06T17:19:17.130
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2017-02-06T17:19:17.130
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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.
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2017-02-06T20:42:43.813
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2017-02-06T20:42:43.813
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answer