Used to give a reason or make an excuse, often with a nuance of complaint, justification, or explaining an unavoidable situation. It's a colloquial form of '〜ものだから'.
Verb [dictionary form] + （も）んだからPrimary
い-Adjective + （も）んだから
な-Adjective + な + （も）んだから
Noun + な + （も）んだから
You use this to give a reason for something. It can sound like you are complaining. Or you are making an excuse.
だってお腹がペコペコだったもんだから、ケーキ全部食べちゃったんだ。
Well, I was starving, so I ended up eating all the cake.
全然お金がないもんだから、今週末はどこにも行けないよ。
I have no money at all, so I can't go anywhere this weekend.
「なんで遅れたんだ？」「すごい事故だったもんだから…」
Why were you late?' 'Well, there was a terrible accident...'
今日は暑いもんだから、何もする気が起きない。
It's so hot today, that's why I don't feel like doing anything.
試験がすごく大変だったもんだから、あまり出来なかったと思う。
The exam was really tough, so I don't think I did very well.
This is different from から and ので. Those just state a reason. This phrase is used when you make an excuse. It is also softer than 〜せいで. That phrase blames someone. This one just explains your situation.
Don't use this when you talk to your boss. It sounds like you are making excuses. For example, don't say "眠かったもんだから遅刻しました" (I was sleepy, so I was late).
Use this when you want to give a reason for something. It helps people understand why you did it. It shows the reason was strong or unavoidable.
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