べき - Grammar Discussion

must do
should do


  • Verb + べき

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@mrnoone For this grammar, can you update the reviews so that you have to input the whole answer, including the verb?

For example:


It was a little odd that this sentence included the す and you only had to fill in べき. I think it would be better to have to input すべき. If you specifically wanted to check that we know the shortened version すべき then perhaps you can just flag and not accept するべき if it’s given as an answer.


I will change it in few hours :slight_smile:

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@seanblue I have updated the reviews to require the whole answer, including the verb. Cheers!


I see that you want “must”, and so I gravitate immediately towards. “~なくちゃいけない”.
I understand that ~べき can mean must, as in “it’s a social obligation, and while it’s technically a ‘should’, it’s really a ‘must’”.
Would it be possible to program this one to give a gentle “non-wrong” answer to ~なくちゃいけない, nudging the user towards べき?

Otherwise, I feel like at this point I’m remembering that “Bunpro wants this particular example sentence to use 〜べき” rather than learning applicable rules about Japanese.


Of course I will add it to my daily to do list. Remember however that 〇〇いけない are stronger than べき。


Thank you so much for your consideration, and for your tip.


I changed all translations to “should” and added 〇〇いけない/ならない answers with notifications.

Of course only to questions where those can be used. When input is followed by だ (but です can follow it), or where there is no verb to modify - it cannot be used in the first place.

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One of the examples includes this sentence:
What is the な doing in this case? I wouldn’t have expected のだ because it sounds off [Edit: Shortly after I said this I came across it in an example, so never mind that part], but I might have expected んだ alone. The な makes it sound like a question to me, but presumably it doesn’t to a native speaker?

It’s just doing the usual harmless copula things before the の/ん.
This structure is covered in んです.
The sentence is not trying to say “it’s the leader’s!” (リーダーのだ).

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Thanks. So the な completely changes the meaning, then - it’s not purely grammatical?

Edit: Just had it in my reviews again and it made me think. So, presumably the な makes it literally “It is the leader” instead of “It is of the leader”, and the context makes that “You are the leader”. Is that right?

What is the difference between…

1・知っておくべき <—- this one is used in an example sentence here on Bunpro



I tried looking the others up but I couldn’t find much.
I’m not even sure if 2 and 3 are even grammatically correct

Thanks for the help

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The suffix ~ておく gives the impression that the verb is in preparation for something, so your first example is ‘~should [prepare by] knowing ~’.
Fun fact: you can abbreviate ておく to とく in an informal setting!

Your second example is a simple ‘~should know~’

Your third example is also ‘~should know~’ but this particular verb isn’t generally used in the plain form. Can anyone elaborate on when it is?


Wow! Thanks for the fast response!

It seems like 知っておくべき and 知ってるべき are the two points we can use!


Relevant research I’ve already tried

The Information section begins with this background:

However, looking at my ‘go to’ reference Jisho.org (which may not be the best overall, I don’t know, but it’s the best thing I know how to use well), I looked up the kanji 可 in conjunction with the kana べ (using this search text: *可* *べ* ), and found three relevant listings for the ‘origin’ of this formation:

  1. , listed as a ‘generic’ word type of Auxiliary meaning:
    • should; must; ought to
  2. , listed as also as a generic Auxiliary meaning:
    • must; should; ought to
    • will (surely); be bound to; must
    • will (do); shall​
  3. , listed specifically as Auxiliary verb and Conjunction, meaning:
    • in order to; for the purpose of
      But also as a generic Auxiliary and Suffix, meaning
    • must; should

So, from this information, it appears to me that the ‘true’ Auxiliary verb would be
べく (seemingly a く verb, which seems to make sense), and the other two forms べき and べし having been derived from べく. [Seemingly べき being derived from べく makes sense, changing the くto a き as usual. Not sure how べし would have been derived. Is there a missing (obsolete?) べす in the mix?]

Main question

So, I’m really just wondering if the background info is etymologically correct or not. Is べき really considered an auxiliary verb, and it being the attributive form of べし (presumably also an auxiliary verb)?

Or is it perhaps (as I suspect, but obviously could be wrong) that the original auxiliary verb is べく, and maybe べき is the attributive form of that? And then べし has some other relation to either べくdirectly, or maybe to べき somehow?

Reasoning behind asking such an esoteric question

For me, I find these kinds of ‘origin stories’ (especially if they are the true origin stories, and not something I accidentally ‘made up’ when trying to piece things together) to be very helpful for me to remember not only the words and phrases themselves, but also how to use them. And they often also give me deeper insight into how the Japanese language as a whole has a kind of ‘logical sense’ to it, at least in terms of how it evolved to become the way it is today. Like, “X leads to Y leads to Z, even if we mostly use Z today, and X is only rarely used (such as in set-phrases) today.”

So, getting the ‘correct’ or ‘official’ story behind these origins and interconnections is very helpful for me when using BunPro. Hence why I’m spending such a long comment on what might seem to be a rather trivial point of detail.

Thanks in advance!

These are conjugated forms of the same word. It’s theorized that the origin is the adverb うべ transformed into an adjective. べし is the 終止形, べく is the 連用形, and べき is the 連体形. It somehow avoided the changes that happened since then and in modern Japanese, these classical forms are still used, but some conjugations have been (partially) replaced with ones that act like it’s a noun (べきだ、べきではない etc.). “Partially” because e.g. べからず is also still used as a negative form. But others are not, e.g. べかりき、べかりけり past forms are not used anymore.

As for whether it’s a verb or not… in Japanese grammar terms it’s a 助動詞 which could indeed be translated as “auxiliary verb”. But 助動詞 don’t necessarily act like full verbs, e.g. -た、-たい、-ない are 助動詞 too. So maybe it’s better to call them “auxiliaries”. The term has nothing to do with their origins anyway, it just describes what they’re used for now.


The description says that this grammar point “implies some sort of moral obligation, or that (A) is the only reasonable course of action.”. I can see this in most of the examples, but some of them don’t feel like that to me:

  • この茶碗べきだ。こんなはあまりないよ。
    You should buy this bowl. It is not often that you come across such a bargain.
  • 日本語してべきだ。その上達するから。
    You should read Japanese aloud, because you will get better that way.
  • 日本語っておくべき 情報がこのにはたくさんかれている。
    This book has a lot of information that people who learn Japanese should know.

I got the first sentence yesterday, and the second sentence today - and I failed both, thinking it would be たほうがいい, といい or たらいい・といい (which I just learnt too). Since I don’t understand what makes them べき, I’ll probably keep failing them. Why are those examples べき?