When to use べき and when to use べし


From my meager understanding of Middle Japanese, I have learned that, in
essence, べき is the attributive form and べし is the predicative form (feel free
to correct me if I'm wrong), but in Modern Japanese, I've definitely seen
evidence that contradicts this information. Can you just use whichever one you
want? What are the rules?




In modern Japanese, べし is not used except for in some fossilized expressions.

As for べき:

  1. Attributively:

べき is still used, although with a slight dated feel to it. In colloquial
contexts it would often be expressed as ～ないといけない instead.

  2. Predicatively:

べき is used as a noun:

> 行くべきだ [sby] should go

Or you could say that べきだ is a verbal phrase. I think it makes more sense to
analyze the べき without だ as copula-dropping than an actual predicative use of
べき by itself.

Note that in front of the formal noun の, the noun version is used, not the
attributive version

> x 行くべきのだ  
>  o 行くべきなのだ

