How can I say ' my mom sick today ' in Korean
Plz informal way ... I want to tell that to my friend

Question

Answer

오늘 우리(나의)  엄마가(어머니께서)  아파(아프셔)

Answer

오늘 저희 엄마께서 편찮으세요. It's pretty politeness. (formal)
오늘 우리 엄마 아파. (우리엄마 오늘 아파.) (casual)

Answer

Informal towards your listener and your mom ; 오늘 우리 엄마 아파.
Informal towards your listener,but respectful towards your mom ; 저희 어머니는 아프셔.

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If you googled it it will come up with - 엄마가 오늘 아프 
My translation is this. 엄마가 아픈 오늘 ..

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Also still learning! I don't know if it's correct thou.

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아프다 is probably fine.. but for all senior and older generation it is better to use 편찮으시다 instead.