This pattern expresses a strong moral or practical judgment that something should not be done. It is more forceful than 'ないほうがいい' (better not to) and carries a sense of obligation or ethics. For example, '遅刻するべきではない' means 'You should not be late' — it implies that being late is wrong, not just inconvenient.

It is formed from the verb 'べき' (which comes from the classical 'べし') plus 'ではない'. When using nouns, you need 'である' before it: '学生であるべきではない' (should not be a student) — but this is rare. More common is with verbs. In speech, it sounds strong and a bit preachy, so it is often used in advice, warnings, or societal commentary. For personal advice, 'ないほうがいい' is softer.

Note: The negative form 'べきではない' is often shortened to 'べきじゃない' in casual speech, but be careful — 'べきじゃない' can also be used with a softer tone. Stick with 'べきではない' for formal or written contexts.