English translation:
because, cause
Structure
Verb + (んだ) + もの/もん
Noun + (なんだ) + もの/もん
いAdj +（んだ + もの/もん
なAdj + (なんだ) + もの/もん
Explanation:
[indicates reason or excuse・can indicate dissatisfaction・very casual speech・if speaker wants to be more polite, he/she can use んですもの instead]
Why is もんだ/ものだ marked as wrong? According to your reading links it is all the same.
Hey 
Sorry for the late answer 
もの if used to state reason doesn’t take だ unless it’s used in the subordinate clause. （素人が創作したもので、やっぱりすぐに壊れてしまった。）
This is one of the cool methods you can use to distinguish different uses of もの on the JLPT.
So what’s the rule for having / not having to put “nanda” in front of “mon” ? Sometimes just “mon” is marked wrong and sometimes not, but I can’t think of a reason
I just learned in class, using the Try JLPT3 book, that もの (as reason/cause) can also be used with ですから or even だから (although it will be もん then), which doesn’t seem to fit with your reply here, nor with the info on the meaning page for this grammar. That doesn’t entirely follow your subordinate clause exception, or does it? Every example sentence in the book had the から sentence/clause first and then either trailing dots or a comma followed by a second sentence/clause.
Since から can be followed with a 。. I haven’t thought of it as a subordinate clause since it can stand by itself.
Hey 
You are absolutely right, ものだから、もので、ものだから can all be used to express reason/cause. If those are used at the end of the sentence, instead of もの(もん) by itself, then clause has been omitted. That’s why there are dots in TRY N3 books.
(By the way, you can use もん with ですから and もの with だから, it is a matter of style, もん is more casual and sounds a bit childish). もの(もん) by itself appears in TRY N2 book.
Another thing I wanted to express but wasn’t clear about is when expressing reason, “だ” by itself is not attached to もの. In other words “Aものだ。” is not used to express reason.
This is an easy way to distinguish it form other uses of もの (it has plenty of them) that use “Aものだ” pattern.
That is:
By the way, notice when making もの (without だ after) polite です appears before it. So:
「美味しいですもの。」
While when you make ものだから more polite it appears after もの
道が渋滞していたものですから …(時間通りに来られませんでした)。遅れて来てすみません。
I hope it clears everything!
Sorry for the confusion 
So basically look carefully when だ/です comes after もの because depending on what is happening around it, it might not mean the same thing. While I guess if もの comes after だ/です it seems like it will always be reason/excuse/etc?
Hey guys! I’m just throwing my two cents here.
The thing is that this usage of もの is fundamentally emotive, like putting in a bit of attitude. As the resources cite, Tae Kim explains it in a really cool way.
どうしてこなかったの？
Why didn’t (you) come?授業があったの。
(I) had class. [feminine explanatory]授業があったもの。
(I) had class. [feminine explanatory]授業があったもん。
(I) had class, so there. [feminine explanatory]
Also, A Dictionary of Basic Japanese Grammar (Page 257) has an entry and really interesting notes on this:
“The speaker presents some situation as if it were a tangible object.”
“Because the speaker presents a situation before もの as if it were a tangible object, expressing different degrees of emotion.”
So cross-examining those two sources, it really is as if you were “conjugating” in emotion I think!
Good news everyone.
We can add a video from Japanese Ammo with Misa (Make Excuses & Justify Yourself Like the Japanese もん/もの/んだもん！JLPT N2 - YouTube) in the readings for this grammar point and also https://bunpro.jp/grammar_points/346
The Information section of the grammar page needs to be cleaned up. The words “because” and “cause” are not the same meaning. When “because” is shortened in casual conversation, an apostrophe is used to show the contraction:
do not → don’t
should not → shouldn’t
because → 'cause
Without the contraction apostrophe, the word “cause” is a different meaning.
'cause Definition & Meaning - Merriam-Webster
Cause Definition & Meaning - Merriam-Webster
Because Definition & Meaning - Merriam-Webster
(notice the Synonym section for “because” includes “'cause”)
Is なのだもの wrong? Because it sounds unnatural?