べきではない means "should not": doing this would be wrong, as a matter of principle. It follows the dictionary form of a verb, and the negative sits on べき rather than on the verb. With , both and the more written are correct.
。
You shouldn't tell lies.
。
You shouldn't break promises.
。
You shouldn't judge people by how they look.
The polite form is べきではありません. In speech you hear べきじゃない.
。
You shouldn't say things like that.
べきではない or ないほうがいい
is practical advice: it would be better for you not to. べきではない says the act itself is wrong, so it suits rules of conduct and sounds like a lecture when aimed at one person.
after it turns it into your opinion.
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You'd better not go out today.
。
You shouldn't smoke in front of children.
。
I don't think you should carry it all on your own.
べきではなかった
In the past it is "shouldn't have": it was done, and it was a mistake.
、。
I shouldn't have left him alone back then.
。
I shouldn't have said such a terrible thing.
Lingaku