Unit 4 - Section 4: 〜ものだ
〜ものだ (how things are / how one should / how I used to)
ものだ comments on a general truth, a social expectation, or a recollection. Tone comes from context, not from one English gloss.
人は誰でも間違えるものだ。
People make mistakes. (general truth)
借りたものは返すものだ。
You should return what you borrow. (that’s how it is done)
子供のころ、よく川で遊んだものだ。
When I was a child I would often play in the river. (recollection)
ものだ after a dictionary or た-form. んだ／のだ is a different “explanatory” ending.