ものだ can be used in two main ways. First, to state a general truth or social norm, like something that is naturally expected. For example, 人間はいつか死ぬものだ means 'humans eventually die' — it's a universal fact. This use often carries a philosophical or resigned tone. Second, with past tense verbs, it expresses nostalgia or fond memory of repeated past actions. For instance, 子供の頃よく母と散歩したものだ means 'When I was a child, I often took walks with my mother (and I remember that fondly).' It's not just stating a fact — it's reminiscing. In both uses, ものだ adds emotional weight, so it sounds more heartfelt than a plain statement. It is common in written language and formal speech, but even in casual conversation you can hear it, especially when someone is being reflective.