What does んだもん mean and is used for?

Question

I am currently studying for the JLPT N3 exam and I come across the following rule that I don't really understand. んだもん

Although it is not identic, according to this link ~もん is

Used to reply dissatisfaction in very casual speech

Which kinds of makes a lot of sense according to the following example from the book：

「どうして食べないの」「だって、まずいんだもん」　(日本語総まとめ　N3 p.71)

"Why didn't you eat?" "Because it tastes awful."

However, I don't see how it applies to the following example:

今日の試験、できなかった...。勉強しなかったんだもん、仕方がない。

I didn't do well on the test today. I can't complain because I didn't study much.

What is this rule actually used for and how does it take/make sense?

Extra information and links are welcomed

Thank you for your time.

Edit 1: Corrected the mistake mentionned.

Answer 1

Although んだ and もん are often used together, it's better to think of them as two separate grammatical elements.

んだ is a colloquial version of のだ, and this の is known as explanatory-の.

• What is the meaning of ～んです/～のだ/etc?
• Wasabi: Explanatory のだ （んだ）

もん is a sentence-end particle that adds the nuance of "hey!", "come on!", "listen!", "you know", "I mean it", etc. Note that this sounds childish. It's used like so:

• 見たもん！ (Hey,) I did see it!
• 行くもん！ (I insist,) I'll go!
• できるもん！ Come on, I can do it!

Put together, んだもん is used to emphatically describe a reason with emotion, like "Come on, it's because ～", "You know, ...", etc.

Answer 2

If your translation "I can't complain because I didn't study much." was correct, maybe the か in 勉強しかなかった was a typo? If so, the sentence would be natural. Below I assume this was the case:

"Dissatisfaction" is a translation that often works, but there is no single one-to-one translation. In the 勉強 example, the meaning is to emphasize that <​while possibly regretting not having studied>, admitting that the poor success was caused by ones own lack of studying, and that the cause-effect was obvious. The もん stresses the obviousness, ie could also be translated as "what else can you expect if you don't study"