What's the difference between すべき (subeki) and べき (beki)?

すべき (subeki) = should do; ought to do

べき (beki) = should; must; that one ought to; to be done

Both mean "should", but is there a rule as to when to use one instead of the other?

Here are some examples from my text book:

motto sentakushi ga aru beki dato omoimasu.
I think there should be more choice

Nani o subeki ka wakarimasu yo.
You'll know what to do, I'm telling you.

What's the difference between the two? In the examples above, they seem to perform the same function. So why is one used over the other?

  • 2
    How did you determine that "both mean 'should'"? A dictionary lookup should tell you that すべき is essentially する+べき, no? Some basic research would improve your questions.
    – user1478
    Commented Dec 10, 2016 at 22:03
  • Did you look in any dictionary? Even WWWJDIC (e.g. jisho.org) has that すべき=する+べき. Once you know this, what is your question?
    – Earthliŋ
    Commented Dec 10, 2016 at 23:27
  • @Earthliŋ @Snailplane. I did check in a dictionary, which lists "should do" for するべき and "should; must;" for べき. So they both mean "should" as stated in my question. But the dictionary doesn't make it clear when to use one over the other (Hence, I put is there a rule as to when to use one instead of the other? in my question). How do you know when to use one instead of the other. It's not very clear.
    – big_smile
    Commented Dec 11, 2016 at 7:50
  • 1
    @Snailplane I have updated my question to make it clearer.
    – big_smile
    Commented Dec 11, 2016 at 8:29

1 Answer 1


べき is used with a verb in its "dictionary form".

should [verb]

As you can see, べき by itself corresponds to "should", but it never appears by itself (just like "should" doesn't appear by itself).

You can easily form examples,

食べるべき = should eat
行くべき = should go

You only need to remember one additional rule,

する+べき → すべき

(About one in twelve times people also use するべき, probably in contexts where する is felt to be used for more than its usual auxiliary function, so don't be surprised if you see it.)


[verb]べき = should [verb]

するべき ≒ すべき = should do

In one line, you don't choose between べき and すべき, you only choose a verb to go with べき and if it's する, then it becomes すべき.

  • As I understand it, する indicates 'to do'. So in my example: Nani o subeki ka wakarimasu yo.: Does this use subeki because it has the meaning of to do (even though する doesn't appear in the example) Is that correct?
    – big_smile
    Commented Dec 11, 2016 at 11:05
  • 2
    Almost. In the example "nani o subeki ..." the verb suru does appear, but in shortened form, because suru + beki combines to subeki (this su is from suru).
    – Earthliŋ
    Commented Dec 11, 2016 at 11:15

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