I have read that verbs in Japanese are a closed class. That is to say: not counting inflections, there is some relatively fixed number of "true verbs" in the language. By "true verbs" I am excluding compound verbs formed with [noun]+する, or really [noun]+[verb] of any type. If this is the case, surely there is some estimate of roughly how many such verbs exist? I believe that i-adjectives are also a closed class, and I'd be curious about their number as well.

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    I’m voting to close this question because it is too broad. I don't think there's any way to know this.
    – istrasci
    Sep 1 '20 at 18:47
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    Something is either 'fixed' or it isn't. Saying 'relatively fixed' doesn't make sense when you are asking if something is either closed or not, to be honest. As pointed out above, there are new verbs and adjectives being added, demonstrating that neither are a closed class. You can't accurately quantify an ever-changing quantity.
    – kandyman
    Sep 1 '20 at 19:02
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    I'd disagree with that literature then. New verbs are not that uncommon. Consider クッキングる ("to cooking", intentionally odd), スタバる ("to go to Starbucks"), ヤフる ("to search for something on Yahoo"). Verbs are not a lexically closed class in Japanese. There are not tons of new entrants, simply because most verbal concepts already have a word for them. But as new verbal concepts arise, so too do new verbs to express those concepts. Sep 1 '20 at 21:24
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    "Closed" word classes generally don't include productive morpho-syntactic elements. In this case, Japanese allows the relatively easy and productive formation of new verbs by tacking ~る on the end, and new adjectives by tacking ~い on the end. The ease of such word formation makes it unlikely that either class will be closed. By comparison, the catch-all 助詞 part of speech sees new entrants only rarely, and can more easily be called "closed" -- certainly not absolutely (even the common particle から "because" developed during the historical era), but definitely more so than verbs or adjectives. Sep 1 '20 at 21:27
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    There are many recent, non-loaned verbs and i-adjectives including ばえる, じわる, もやる, きょどる, ぼこる, もにょる, 告る, 事故る, げすい, ぱない, かすい, ちゃらい and ちょろい. Sanseido's "新語 of the Year" often includes such new words. I kind of feel i-adjectives and verbs are becoming more and more productive these days. I even have a handful of i-adjectives usable only within my family :D By the way, compound verbs usually refer to those lexicalized [verb]+[verb]'s such as 動き回る, 飛び立つ and 聞き流す. Can we assume you want to exclude them, too?
    – naruto
    Sep 1 '20 at 23:06

Coining new godan verbs (u-verbs) is far from exceptional in modern Japanese, and we have many related questions on this site.

I listed some well-known ones in the comment section, but just to support this, here are some rare u-verbs I happen to know personally:

  • ジャムる: "to be jammed" (gun, military jargon)
  • バベる: "to transpile JavaScript source files using Babel" (programmer's jargon)
  • ばみる: "to use plastic tape to mark the positions of actors on stage" (actor's jargon)
  • メタる: "to defeat an opponent with the aid of metagame" (card gamer's jargon)
  • アポる: "to develop a brain infarction" (medical argot)
  • サチる: "to saturate; to reach the limit" (data scientist's jargon)
  • マミる: "to be decapitated" (anime fan's jargon)
  • タヒる: "to die" (from the bottom part of 死)
  • へごる: "to say something funny unintentionally" (Ayaka Ohashi's fan's jargon)
  • ジュレる: "to be half-frozen" (leaves, gardening jargon)

Okay, these are used only in small communities, and I think less than 10% of native Japanese speakers know them. But I believe words like these are coined almost every day somewhere in Japan, and this is exactly why u-verbs are productive in Japanese.

On the other hand, the number of Japanese 助動詞 is relatively small and you can see the full list in Japanese Wikipedia.

Anyway, if you want a rough estimation, according to this page, the number of Japanese verbs listed in a certain dictionary is 10,265 (but this should include lexicalized compound verbs like 取り戻す). 国研日本語語彙DB recognizes 236 i-adjectives, but according to this page, the number of Japanese i-adjectives recognized by ATOK9 is much larger (1,302). 『品詞別日本文法講座4 形容詞・形容動詞』 by Suzuki et al lists 1,343 i-adjectives (and archaic precursors).

EDIT: I should've mentioned this... Ichidan (ru-) verbs are "relatively closed". According to this page, there are only 40 ichidan verbs that end with -iru (excluding compound verbs and archaic verbs).

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    I’m really surprised ジャムる is rare, and that you’ve listed it as military jargon. I heard it many times in office settings in Japan in the 1980s, always in reference to printers and copiers.
    – Nanigashi
    Sep 2 '20 at 19:08
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    @Nanigashi Personally I have heard only 詰まる for such situations, but there can be workplaces that have used ジャムる since the 80s. So this is another reason why u-verbs are "open" :)
    – naruto
    Sep 2 '20 at 21:13
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    Yes, exactly! :)
    – Nanigashi
    Sep 3 '20 at 20:54

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