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
Wakai toki wa takusan asobu mono da.
It is natural to have a lot of fun when you are young.
Yakusoku o shitara, mamoru mono da.
If you make a promise, you should keep it.
Okyakusan ga kuru to, ocha o dasu mono da.
When guests arrive, it's customary to serve tea.
Jinsei wa konnan ni tachimukau mono da.
In life, one ought to face hardships.
Practice
Grammar content sourced from hanabira.org.