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A girl forgot to do her homework and mum catches her out of bed:
「まる子っ、もう寝なさい」 Maruko, go to bed now!
「うん、もうすぐ寝るから」 Yes, I'll go to bed very soon (because)
I don't understand the function of から here. At first I thought it should be an unfinished excuse for why she is out of bed, but I thought that would be もうすぐ寝るが... Then I thought it could be "I will go to bed soon (rather than now) because (I have to finish my homework), but から is on the wrong clause for that to work. I'm out of ideas.
「うん、もうすぐ[寝]{ね}るから。」
「から」 here is used like a sentence-ending particle, and that is one very common usage of the word in informal speech.
We use 「から」 this way to make an announcement and see how the other person would react. More often than not, the speaker simply expects that reaction to be along the lines of 「わかった」、「じゃあいい」、「それならいい」、etc., which would be the Japanese equivalents of "Alright, then!" or "Okay, then!" In other words, this 「から」 is not really used when one knows that one's statement would cause a further argument.
"Right, I'll hit the sack in a minute!"
In this particular line by Maruko, the nuance of 「から」 would be like saying "Don't you worry!" without saying it.
In other situations, however, the から-ending can be used to make stronger statements of warning, determination, etc. as in:
「[死]{し}んでも[許]{ゆる}さないから！」 "I won't forgive you even if I die!"
「ママに[言]{い}いつけてやるから！」 "I'll tell Mom on you!"
EDIT: Similar example sentences can be found in Definition #3 in this dictionary as well.
