Grammar JLPT Level N3 - Lesson 7

Structure of ものだ:

Verb + もの⁽¹⁾ + だ⁽²⁾
［い］Adjective + もの⁽¹⁾ + だ⁽²⁾
［な］Adjective + な + もの⁽¹⁾ + だ⁽²⁾

⁽¹⁾ もん
⁽²⁾ ではない、じゃない

Details about ものだ:

もの (or 物(もの) in its kanji form) is classified as a 形式名詞(けいしきめいし) (dummy noun) in Japanese. This just means a noun that is used in a similar way to 'thing' in English. もの regularly refers to things that do not have their own name, or are difficult to name due to their complexity. When もの is combined with the auxiliary verb だ, it implies that the previously mentioned (A) is something that 'is what it is'. This is often used in relation to customs, conventions, common sense, or to highlight results that would naturally be expected in any given situation.
To use ものだ, attach it to the end of the attributive form of any word/word that ends a phrase that you would like to highlight as being an obvious piece of information.

仕事(しごと)は一生懸命(いっしょうけんめい)するものだ。
Work is something you should put all your effort into.
ユウキが自分(じぶん)からプランを立(た)てるのは珍(めずら)しいものだ。
It is rare for Yuki to plan something out on his own.
こどもは元気(げんき)なものだ。
Kids should be energetic. (Man, kids are energetic)

ものだ as 'supposed to', or 'the way it is' can also be used to negate a phrase, and say that it is 'something that isn't expected', or 'shouldn't be the case'. To do this, だ will be replaced with ではない, or じゃない.

人生(じんせい)はそんなに簡単(かんたん)な物(もの)ではない。
Life is not something that is easy.
家族(かぞく)は捨(す)てるものじゃない。
Family is not something you should throw away.

ものだ

Usage Notes for ものだ:

もの may sometimes be abbreviated as もん. This is far more common in spoken language, and can make the statement sound a bit more casual overall.

東大(とうだい)に入(はい)れたのか？大(たい)したもんだ。
You were able to get into Tokyo University? That is impressive.
ゴミを道(みち)に捨(す)てるもんじゃない！
You should not throw garbage on the street!