At a glance
～ものではない means "one should not do ~" as a matter of common sense or social norms. It is usually used to correct or warn someone, often by an older person speaking to a younger one.
Verb (dictionary form) + ものではない
人の悪口を言うものではない。 One should not speak ill of others.
How to form it
Attach ものではない to the dictionary form of a verb. The verb stays affirmative, and ものではない adds the "should not".
| Attaches to | Example | Register |
|---|---|---|
| Verb (dictionary form) + ものではない | 言うものではない | Plain, written or firm speech |
| Verb (dictionary form) + ものではありません | 言うものではありません | Polite |
| Verb (dictionary form) + もんじゃない | 言うもんじゃない | Casual spoken |
For this meaning it is used with verbs, not with nouns or adjectives.