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A Japanese grammar book will tell you that to make a 形容動詞(な adjective) into an adverb, you add に after it. For example:

静かに歩いてください

そういうのって確かに非常に辛いな

However, recently, I’ve started to notice sentences where instead of に, で is used. For example:

高速で車を動かしてください

早速で作ってみましょうね

This doesn’t make sense with me. It does not seem to be the で that marks “method” either. For me, it clearly should have been

高速に車を動かしてください

早速に作ってみましょうね

Question: is the usage of で here grammatically incorrect? If not, then what’s the explanation of it(why not に, etc.) ?

Note: this question is SPECIFICALLY about the use of に and で after 形容動詞. Not just their general difference.

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    Where did you see 早速で?
    – aguijonazo
    Commented Aug 12, 2022 at 0:30
  • I think 早速 is a 副詞, not a 形容動詞. It's listed as a 副詞 in 明鏡国語辞典. 「早速なご対応ありがとうございます」ではなく「早速のご対応ありがとうございます」と言うと思いますし。
    – chocolate
    Commented Aug 12, 2022 at 1:39

1 Answer 1

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Question: is the usage of で here grammatically incorrect?

As far as 形容動詞 (な adjective) is concerned, yes.

That said,

高速で車を動かしてください

can be a grammatically correct sentence (if a bit an awkward one), if you see 高速 as a noun, short for 高速道路, and で as a location marker. That is different from the adjective 高速な.

な-adjective's で-form, or continuative form (連用形), is used in different context. For example, you use it when you connect two or more adjectives as in 高速で正確な作業 ("fast and precise work"). The examples given in the question are all adverbial.

In general, many nouns and na-adjectives share some of the forms superficially, but need to be parsed differently depending on the context. 自然 is another example that can be a noun and the stem of a na-adjective.

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    I would also understand this 高速 as a noun, but one meaning something like 高速ギア, 高速モード, or less specifically 高速な状態.
    – aguijonazo
    Commented Aug 12, 2022 at 1:09
  • Thanks. I think I understand now.
    – dvx2718
    Commented Aug 12, 2022 at 2:38

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