Well, because!
明日仕事だって。
あした しごと だ って。
She said she has work tomorrow.
Literally: "tomorrow + work / job + だ + って."
そんなの誰だって知ってるよ。
そんな の だれだって しってる よ。
Everyone knows that (kind of thing).
Literally: "that kind of + の + anyone + is knowing + よ.
However, there is another way to use this unique string of kana: When giving excuses.
A:
なんで来ないの？
なんで こない の？
Why won't you come?
Literally: "why + won't come + の?"
B:
だってどうせつまんないもん。
だって どうせ つまんない もん。
Because, it'll just be boring, anyway.
Literally: "well, because + in any case + boring + もん."
Is that example a bit too tricky? Let's zoom in on some of its parts...
どうせ
どうせ means something like "in any case" or "anyway." That might be a bit misleading.
It is not used when changing topics, as in the sentence, "Anyway, I'll call you tomorrow."
It is used like the "anyway" in the sentence, "I'll never pass (the test), anyway."
Perhaps it's not too surprising that we see どうせ in somewhat negative-sounding sentences quite a bit.
For example, let's say that I should be studying for a big test, but when you come into my room, I'm just playing video games. The following conversation ensues...
A:
勉強しなくていいの？
べんきょう しなくて いい の？
Shouldn't you be studying?
Literally: "studying + even if (you) don't do + good + の?"
B:
意味ないもん。どうせ落ちるから。
いみ ない もん。 どうせ おちる から。
What for? I'm just gonna fail, anyway.
Literally: "meaning + there isn't + because. + in any case + fail (=fall) + から (=because)."
↑ Confused by that usage of もん by Person B? Well, lucky for you...
もん
I wasn't able to use もん, which means something like "because," until I was at a very high level of Japanese.
I suppose it's because I was never taught もん in books, classes — anywhere!
It's not particularly difficult, though. While one could argue that it's more complicated than this, to get started we can just think of もん as being a very casual form of から：
A:
なんで来ないの？
なんで こない の？
Why won't you come?
Literally: "why + won't come + の?"
B:
明日仕事だもん。
あした しごと だ もん。
Because I have work tomorrow.
Literally: "tomorrow + work / job + だ + もん."
Technically, もん is a colloquial version of もの, which can also carry the meaning of "because" (we'll see this in future JLPT lessons). Unlike から, in most cases you will only find もん at the very end of a sentence. もん also has the tendency to sound a bit childish — the nuance is that the speaker is explaining something but also pouting.
Anyway, yeah, I hope this makes more sense now:
A:
なんで来ないの？
なんで こない の？
Why won't you come?
Literally: "why + won't come + の?"
B:
だってどうせつまんないもん。
だって どうせ つまんない もん。
Because, it'll just be boring, anyway.
Literally: "well, because + in any case + boring + もん."
If not, then we can just look forward to it making sense someday in the (hopefully near) future!
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