べきMust, Should , Ought to

The auxiliary verb べき, being the attributive form of べし, is a structure in Japanese that is often paired with verbs to express that something 'ought to be done'. It implies some sort of moral obligation, or that (A) is the only reasonable course of action. This comes from the kanji 可(べ)き, which carries the meaning of being something 'permissible' (indicating that any other action would not be acceptable).

べき will be paired with the plain (non-past) form of a verb, before being followed by either だ, or another noun.

べき is also sometimes translated as 'must do (A)', or 'should do (A)' but it is important to remember that it will always have the nuance of being the only 'permissible' choice, and is therefore very strong.

Caution

It is important to note that the verb appearing with べき cannot be in past tense, and you will need to use だった or でした after べき, if you would like to say that something 'ought to have been done' in the past.

Fun Fact

In modern Japanese, there are only 18 auxiliary verbs, of which べき is not one. べし (the standard form) is considered to be a classical auxiliary verb. Due to this, べき (and べし) are often referred to as classical auxiliary verbs.

Fun Fact

The combination of する and べき is a rare grammatical exception in Japanese, in which two forms are both considered equally correct. Due to this, either するべき, or すべき may be used.