Grammar Info
N3 Lesson 2: 17/23

because, cause

Can sound childish/cute

Verb + (んだ) + もの(1)
い-Adj + (んだ) + もの(1)
な-Adj + (なん) + + もの(1)
Noun + (なん) + + もの(1)

(1) もん
Verb + (んです) + もの(1)
い-Adj + (んです) + もの(1)
な-Adj + (なん) + です + もの(1)
Noun + (なん) + です + もの(1)

(1) もん
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使用域 一般
もの, an extremely common noun in Japanese that means ‘thing’, can also be used as a 終助詞(しゅうじょし) (sentence ending particle). In these cases, it adds a small amount of emotion to a statement, and presents it as a reason. Due to this, もの is usually translated as ‘because’, or ‘cause’. Literally, it just means ‘it’s an (A) thing!’. もの is quite casual, so should be avoided in formal situations.
To use もの as ‘because’, attach it to the end of the 終止形(しゅうしけい) (plain form) of any word.
  • あなた言(い)う事(こと)は何(なに)も信(しん)じられないわ。だって、あなたはいつも嘘(うそ)つくもの
    I can't believe anything you say. Cause you always lie.
  • だって、彼女(かのじょ)悪(わる)いんだもの
    Cause she is the one in the wrong.
  • これはあなたにあげられない。大切(たいせつ)だもの
    I can't give this to you, cause it is important.
  • すぐ泣(な)くに決(き)まってるじゃん、まだこどもだもの
    Of course he will cry, cause he is still a child.
To emphasize もの even further, the explanatory んだ (after a verb or い-Adjective), or なんだ (after a な-Adjective or noun) will often be used.
  • エアコンつけて寝(ね)ているよ。だって、暑(あつ)くて寝(ね)れないんだもの
    I sleep with the A/C on. Cause it’s so hot that I can't sleep, you see.
  • この子(こ)は失敗(しっぱい)するに決(きま)ってるでしょうまだ新人(しんじん)なんだもの
    Of course this kid will make mistakes. Cause he’s still new here.
もの can be shortened to もん in its use as a sentence ending particle. However, this makes the statement sound far more ‘cute’ or ‘childish’. Due to this, it should be avoided in standard conversation.
  • もうお腹(なか)いっぱいだもん
    Cause I'm already full.
  • いいよ、お前(まえ)もう遊(あそ)ばないもん
    It's all good! I'm not going to play with you anymore! (Showing mild anger/jokingly)
Fun Fact
Although it is not used regularly in standard Japanese, んですもの is a slightly more polite version of んだもの that appears frequently in anime, manga, and novels.
  • こんなものすぐに壊(こわ)れるに決(き)まっているじゃないですか、紙(かみ)から出来(でき)てるんですもの
    Of course something like this will break instantly. (Because) it is made of paper.
  • しょうがないですよ、あの人(ひと)は何(なに)も考(かんが)えていないですもの
    We can't help it. (Because) that person doesn't think about anything.
Slow Male
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(Because) she is lying.
Don't be angry. (Because) he is just a kid.
(Because) it tastes bad!
Tae Kim's Japanese Grammar Guide
Page 238