〜ものだ
That's the way things are (general truth); how… (exclamation)machine translation
FormationPlain non-past form + ものだ (な-adjective + な)
〜ものだ has two main uses. The first is stating the NATURE, the common way, of things and people: 赤ちゃんは泣くものです (babies cry — that's simply their nature). The speaker isn't discussing one particular case but a general rule everyone accepts.
The second is EXCLAMATION — voicing a deeply felt emotion at some truth: 時間がたつのは早いものですね (time really does fly, doesn't it). In this use, ものだ makes the sentence sound far more heartfelt than an ordinary statement.
Connection: plain non-past form + ものだ; な-adjectives add な. Don't confuse it with 〜ことだ, also learned at N3: ことだ is advice aimed at ONE person in ONE specific situation, while ものだ states a general truth that applies to everyone. In the same family you'll also find 〜ものではない (one shouldn't — by common sense) and 〜たものだ (nostalgia), which you'll meet right after this.
Examples
人間は年を取ると忘れやすくなるものだ。
People naturally become forgetful as they get older.
赤ちゃんは泣くものです。
Babies cry — that's just what they do.
時間がたつのは早いものですね。
Time really does fly, doesn't it.