Grammar JLPT Level N3 - Lesson 7

Structure of もの・もん:

Verb + （んだ） + もの⁽¹⁾
い-Adj + （んだ） + もの⁽¹⁾
な-Adj + （なん） + だ + もの⁽¹⁾
Noun + （なん） + だ + もの⁽¹⁾

⁽¹⁾ もん

Details about もの・もん:

もの, an extremely common noun in Japanese that means 'thing', can also be used as a sentence ending particle. In these cases, it adds a small amount of emotion to a statement, and presents it as a reason. Due to this, もの is usually translated as 'because', or 'cause'. Literally, it just means 'it's an (A) thing!'. もの is quite casual, so should be avoided in formal situations.
To use もの as 'because', attach it to the end of the plain form of any word.

あなたが言(い)う事(こと)は何(なに)も信(しん)じられないわ。だって、あなたはいつも嘘(うそ)をつくもの。
I can't believe anything you say. Cause you always lie.
だって、彼女(かのじょ)が悪(わる)いんだもの。
Cause she is the one in the wrong.
これはあなたにあげられない。大切(たいせつ)だもの。
I can't give this to you, cause it is important.
すぐ泣(な)くに決(き)まってるじゃん、まだこどもだもの。
Of course he will cry, cause he is still a child.

To emphasize もの even further, the explanatory んだ (after a verb or い-Adjective), or なんだ (after a な-Adjective or noun) will often be used.

エアコンをつけて寝(ね)ているよ。だって、暑(あつ)くて寝(ね)れないんだもの。
I sleep with the A/C on. Cause it's so hot that I can't sleep, you see.
この子(こ)は失敗(しっぱい)するに決(きま)ってるでしょう、まだ新人(しんじん)なんだもの。
Of course this kid will make mistakes. Cause he's still new here.

もの・もん

Usage Notes for もの・もん:

もの can be shortened to もん in its use as a sentence ending particle. However, this makes the statement sound far more 'cute' or 'childish'. Due to this, it should be avoided in standard conversation.

もうお腹(なか)いっぱいだもん。
Cause I'm already full.
いいよ、お前(まえ)とはもう遊(あそ)ばないもん！
It's all good! (Because) I'm not going to play with you anymore! (Showing mild anger/jokingly)