〜ものだ
(that's how it is); used to; you should
Plain form + ものだ · た-form + ものだ (used to)
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ものだ has three related uses, all about how things generally are.
General truth: 人は変わるものだ, "people change". It states the nature of things, often with a sigh.
Nostalgia: after a た-form, it recalls a past habit: 子どものころは、よくこの川で遊んだものだ, "as a kid, I used to play in this river a lot". It's fond and wistful.
Social norm: 人の話は最後まで聞くものだ, "you should listen to people until they've finished". It's how people in general ought to behave, so it can sound like a lecture.
With ね, it expresses feeling: 時間がたつのは早いものですね, "time really flies, doesn't it". In speech it's often もんだ.
Examples
人は変わるものだ。
People change.
子どものころは、よくこの川で遊んだものだ。
When I was a child, I used to play in this river a lot.
人の話は最後まで聞くものだ。
You should listen to people until they've finished.
時間がたつのは早いものですね。
Time really flies, doesn't it?
若いころはよく徹夜したものだ。
When I was young, I often stayed up all night.