〜ものだ
One should / used to 〜ものだ
Meaning
Three uses: a general truth, nostalgia for what one used to do, and mild moral advice.
Formation
[plain non-past] + ものだ (truth / advice) · [plain past] + ものだ (nostalgia)
Notes
The tense decides the meaning: 若い頃はよく遊んだものだ is "I used to play a lot", while 年を取れば忘れるものだ is "people do forget as they age". As advice it is gentler than べきだ: 挨拶はきちんとするものだ.
Examples
若い頃（わかいころ）はよく山（やま）に登（のぼ）ったものです。
I used to climb mountains a lot when I was young.
Breakdown
年（とし）を取（と）れば誰（だれ）でも忘（わす）れっぽくなるものです。
Everyone becomes forgetful with age. (a general truth)
Breakdown
挨拶（あいさつ）はきちんとするものです。
One should greet people properly. (mild advice)
Breakdown
子供の頃（こどものころ）はよく母（はは）に叱（しか）られたものだ。
My mother used to scold me a lot as a child.
Breakdown
時間（じかん）が経（た）つのは早（はや）いものですね。
Time really does fly, doesn't it.
Breakdown
⚠️ Often confused with
〜わけだ
That is why 〜わけだ
Draws a conclusion that follows naturally — "so that's why ~", "no wonder ~", "it works ou...
〜わけではない
It is not that 〜わけではない
A partial denial — "it is not that ~", "that does not mean ~". Softens rather than rejects...
〜わけがない
There is no way 〜わけがない
A flat denial of possibility — "there is no way ~", "it is impossible that ~".
〜に決まっている
Bound to be 〜に決まっている
Says an outcome is obvious — "of course ~", "it is bound to be ~".
〜とは限らない
Not necessarily 〜とは限らない
"Not necessarily ~", "it is not always the case that ~".
〜っこない
Could not possibly 〜っこない
A blunt, casual denial — "there is no chance ~", "could not possibly ~".
〜に過ぎない
Merely 〜に過ぎない
"Merely ~", "no more than ~".
〜というものだ
That is what ~ is 〜というものだ
"That is what ~ really is" — the speaker's verdict stated as a general truth.
〜にほかならない
Nothing other than 〜にほかならない
"Is nothing other than ~", "is precisely ~".