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English
No thanks
Chinese
不用,谢谢
Pinyin
Bú yòng xièxie
Literal meaning:
Not Need Thanks
This is a polite way of saying "no thanks", which will work in many situations.
In the sitaution that your Chinese mother-in-law is trying to persuade you to eat some more, you will probably need to employ a variety of expressions before you succeed with convincing her that you actually had plenty already.