文末表現N3
〜もん
because... (playful excuse)
Structure
句子（普通形）+ もん
Explanation
The casual form of もの, used at the end of a sentence to give a reason or excuse, "because...". Used playfully by children or women.
Examples
だって、一人で行くのが怖いんだもん。
Because I'm scared to go alone.
聞いてなかったんだもん。
Because I didn't hear.
Note
もん is the casual contraction of もの, giving a playful reason or excuse.