～ものだ (〜mono da)
Explanation
Expresses something natural or expected; 'it is normal that...', 'should', 'ought to'.
Formation
Verb-dictionary form + ものだ, な-Adjective + なものだ, (Also: Verb-た form + ものだ for recollections)
Detailed Explanation
「～ものだ」 can indicate that something is regarded as a general truth, common sense, or natural behavior. It can also express strong sentiment or emotion. In the past tense (Verb-た form + ものだ), it often shows nostalgia or recollection ('I used to...').
Examples
若い時はたくさん遊ぶものだ。
Wakai toki wa takusan asobu mono da.
It is natural to have a lot of fun when you are young.
約束をしたら、守るものだ。
Yakusoku o shitara, mamoru mono da.
If you make a promise, you should keep it.
お客さんが来ると、お茶を出すものだ。
Okyakusan ga kuru to, ocha o dasu mono da.
When guests arrive, it's customary to serve tea.
人生は困難に立ち向かうものだ。
Jinsei wa konnan ni tachimukau mono da.
In life, one ought to face hardships.
Grammar content sourced from hanabira.org.