〜ものだから・〜もので
because, you see (an excuse)
Plain form + ものだから / もので (な-adj, Noun + な)
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ものだから gives a reason as an excuse or a personal explanation: 目覚ましが鳴らなかったものですから、遅れてしまいました, "my alarm didn't go off, you see, so I was late". It says "it couldn't be helped".
もので is a shorter version, common in polite apologies: 道が込んでいたもので, "the roads were busy, you see". In casual speech, both shrink to もんだから and もんで.
After a noun or な-adjective, add な: 初めてなもので, "it's my first time, you see".
It's not for giving orders or requests: ものだから can't be followed by "so please …". It explains what already happened, usually something you're apologising for or excusing.
Examples
目覚ましが鳴らなかったものですから、遅れてしまいました。
My alarm didn't go off, you see, so I was late.
道が込んでいたもので、遅くなりました。
The roads were busy, I'm afraid, so I'm late.
子どもが熱を出したものだから、会議を休んだ。
My child came down with a fever, so I missed the meeting.
あまりにおいしかったもので、全部食べてしまいました。
It was so good that I ended up eating it all.
初めてなもので、よくわからないんです。
It's my first time, you see, so I don't really understand.