もの / もん

もの at the end of a sentence gives a reason in self-defence: "but it's cold!", "because nobody told me!". It goes after the plain form, and a noun or a な-adjective takes だ first. In speech it is usually もん.

A noun or a な-adjective needs だ or なんだ before it: ×子供もの, ×子供なもの → こども子供だもの, こども子供なんだもの.

With だって and んだ

もの is often paired with だって at the start, "but" or "because", and with んだ before it. Together they make the standard shape of an excuse.

もの or から

から states a reason. もの defends the speaker: they feel they have to justify themselves, and they sound sulky, childish or endearing depending on who says it.

Register

もの and もん belong to casual speech among family and friends. もん sounds more childish than もの. An adult who uses either at work sounds like a child making excuses. The everyday adult version is から or んだ.