Explanation
Used to convey a sense of justification; 'because', 'that's why', 'it's just that'.
Formation
Verb-casual + んだもん, い-Adjective + んだもん, な-Adjective + なんだもん, Noun + なんだもん
Detailed explanation
The ～んだもん grammar point is used when someone wants to give a reason, justification, or explanatory information in a more casual and emphatic way. It can be translated as 'because', 'that's why', or 'it's just that' in English. The formation differs depending on whether it is used with a verb, い-adjective, な-adjective, or noun.
Examples
Okureta nda mon, gomen.
I was late, that's why I'm sorry.
Oishii nda mon, mou chotto tabetai.
It's delicious, that's why I want to eat more.
Tsukarete iru nda mon, hayaku netai.
I'm tired, that's why I want to go to bed early.
Kare wa tomodachi nan da mon, tetsudawanai wake ni wa ikanai.
He's my friend, that's why I can't not help him.
Practice
Grammar content sourced from hanabira.org.