ものだ

もの has a lot of meanings and usages. A lot. and they aren’t even related to each other a lot of the time.
This page is going to talk about one particular pattern; ものだ and its 3 (main) usages:
Natural fact, Common Sense, and Emotion

Natural Fact

Natural fact
Dictionary form verb + ものだ
Verb ない form + ものだ

Natural fact ものだ is used to express an obvious, natural statement, such as:
Fire is hot. Water is heavier than air.
Generally speaking, these types of statements should be immutable and generally accepted.
You should be careful when using it about opinions.

Examples

• 火は煙を出すものだ – Fire (naturally) puts out smoke. 
• 夜は終わるものだ – Night does end. (Of course).
• 話はいつか終わるものだ – Stories end eventually. (Of course)
• 時は止まらないものだ – Time does not stop. (Of course). 

Common Sense

Common Sense
Dictionary Form Verb + ものだ / ものではない

Common sense もの is used to describe things that are either common sense or obligated.
I almost combined these with “natural fact” もの, but the formation is slightly different with regards to negatives.
You can think of this ものだ as ~のがとうぜん or べき。
You can translate it is “Should” or “is a given“.

Examples

• 接客業では敬語は使うものだ – In service business, using Keigo is a given. 
• 授業はさぼるものではない – Classes should not be skipped.
• 人は殴るものではない – People aren’t for hitting. /Shouldn’t be hit. 
• 子供は親に聞くものだ – Children (Should) listen to their parents. 

Emotion

Emotion
Plain form verb/い Adjective + ものだ
な Adjective + なものだ

The “easiest” ものだ here, this もの expresses emotion.
Typically this is amazement, shock, disappointment, depression, etc.
Has a tendency to be used with なる, 本当に, and adjectives.

Examples

• 子供は本当に元気なものだ – Children sure are energetic. 
• 最近忙しくなったものだ – I’ve really become busy lately. (Sigh)
• 子供は本当に早く成長するものだ – Children really grow fast.
• この町は本当にすたれたものだ – This town has really dried up.

You should use 人 when talking about a specific person:

• 彼は優しいものだ△ – He is a really nice (thing) -> 彼は優しい人だ.

With よく and passive

Often times, you’ll hear this ものだ with よく.
This stresses the statement further.
With よく + (Intransitive) passive (usually いられる) it is a very critical statement.

Examples

• よく食べたものだ – I’m amazed you ate it. (Was hard to do, maybe bad)
• よく言えたものだ – I’m amazed you could say (Was hard to do, probably bad)
• 彼はよくそこに座っていられたものだ – I’m amazed he kept sitting there (Without running away).
• よく来られたものだ – I’m amazed you came. (How dare you OR There’s a tsunami outside).