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When did 役に立つ lexicalize to 役立{やくだ}つ?

Has 役立つ always been a word, and a word that is used more frequently than 役に立つ? A quick google shows 役立つ is used 3 times more frequently.

I am reading the translation of a speech. The translator always says 役立つ where I would say 役に立つ. Is there any connotation difference between the two? Maybe 役立つ less formal? My feeling is that initially there could be some association of lexicalized words with "slang", but as time passes that perception goes away.

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Please refer to the other answer(s) for your main question (etymology).

As for the modern usage, the difference in frequency is not that large. Here are the hit counts from BCCWJ.

  • 役に立 2302, 役に立つ 1039, 役に立っ 270, 役にたつ 94
  • 役立 3031, 役立つ 1419, 役立っ 447, 役だつ 30

役に立つ and 役立つ are not always interchangeable. As a simple predicate, 役に立つ is the more common expression, and saying 役立つ sounds formal/literary/stiff.

  • この辞書は役に立つ。 (perfect)
  • この辞書は役立つ。 (sounds literary/stiff)

But 役立つ is preferred regardless of the formality, when another ~に precedes to indicate the "type of usefulness". According to BCCWJ, roughly 80-90% of the instances of 役立つ are preceded by such ~に.

  • この辞書は勉強に役立つ。 (perfect)
  • この辞書は勉強の役に立つ。 (also perfect)
  • [?] この辞書は勉強に役に立つ。 (weird)

~に + 役に立つ is allowed in expressions like 「間接的に役に立つ」「私にとって役に立つ」「文法の勉強をするときにも役に立つ」, but the vast majority of the 役に立つ examples are not preceded by に at all.

So 役立つ as a simple predicate is not as common as you might think.

  • Please refer to other question(s) って、「他の人の回答を見てください」(the other answer(s)) って書こうとされました?つまりEiríkr Útlendiさんの回答を見てください、っていう…? – Chocolate Jun 29 '16 at 8:56
  • @chocolate お、仰る通りです…すみません – naruto Jun 29 '16 at 13:49
  • Thank you for going into the differences in usage and grammar - this is an important point. – Eiríkr Útlendi Jul 3 '16 at 0:41
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My copy of Shogakukan's 国語大辞典 gives a quote from 狂言記{きょうげんき} using the 役立つ form, dating this term to at least the late 1650s.

As to formality, a native speaker would be able to answer more authoritatively, but I am not aware of any particular difference in register between 役立つ and 役に立つ, and none of my resources to hand indicate any such difference.

  • 3
    私の感覚では、「役に立つ」のほうが口語的で、カジュアルな/日常の会話ではよく使う感じがします。どちらかというと「役立つ」のほうが文語的・フォーマルな感じがします。 – Chocolate Jun 29 '16 at 0:58
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To me both ”役に立つ” and “役立(やくだ)つ” mean the same thing, “It helps,” or “helpful.”

When I consulted how to translate “役立(やくだ)つ” into Japanese with Kenkyusha’s “New Japanese English Dictionary” for a reference, it simply told “See 役に立つ.”

However, it appears to me there's a bit of difference between their usage and nuance. The former (役に立つ) is more often used in “predicative” form in a sentence than the latter (役立つ), which is rather used for “adjective” form in a phrase.

For instances, we say;

[役に立つ]

救急箱はいざという時に役に立つ – The first-aid kit is effective in emergency.

彼は役に立つ人間だということが解った – I found him being an able person we can rely on.

この芝刈り機は役に立たない – This mower doesn’t work.

果たして私、お役に立ちますか、どうか – I’m not sure if I am of any assistance to you or not.

[役立つ]

英語の勉強に役立つ本 – A book helpful in learning English.

就職に役立つコネ – A connection helpful for job hunting.

問題解決に役立つアドバイス – Advice that helps solving the problem.

世間のために役立つ人間になれ - Be a man who is useful for the world.

Though I don’t have any academic ground, I think 役立つ derived from "役に立つ" as its contraction.

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