〜ものではない
One shouldn't (by common sense, morality)machine translation
FormationVerb dictionary form + ものではない
〜ものではない is used to admonish that an action is something one SHOULDN'T do according to common sense and social morality: 人の悪口を言うものではない (one shouldn't speak ill of others). The speaker isn't offering a personal opinion but speaking in the name of shared norms, so this pattern typically appears when parents, teachers, or elders correct those below them.
Connection: verb dictionary form + ものではない. The polite form is ものではありません; in speech it can contract to もんじゃない.
Compared with 〜べきではない (also learned at N3): べきではない is the speaker's fairly strong personal judgment about a specific matter, while ものではない rests on the common sense that "everyone has always understood it to be so," sounding like a general precept. Used with equals or superiors, ものではない easily comes across as preachy — save it for clear senior-to-junior relationships.
Examples
人の悪口を言うものではない。
One shouldn't speak ill of others.
食べ物を残すものではありません。
You shouldn't leave food uneaten.
人前でそんなことを言うものではないよ。
You shouldn't say things like that in front of other people.