How to Say "Thank You" the Right Way in Chinese

I am ashamed to admit it but until recently (very recently) I have been saying "thank you" all wrong.

Both 唔該 ("mh goi" in Cantonese) and 多謝 ("daw jeh" in Cantonese) mean "thank you". So naturally, thinking they meant the same thing (which in my defense, they sort of do), I've been using these two phrases interchangeably. One day, my faux pas was brought to my attention when I was ridiculed at my fiancee's mother's house for saying thank you (唔該 "mh goi") which I thought was the polite thing to do after having dinner at someone else's home. The funny (or sad) thing is that when they tried to explain the difference, I had no clue what they were talking about.

That is why I am writing this post. I hope by explaining the distinction between these two words that have a deceptively similar meaning, I'll finally be able to make sense of when to use what.

This is what I have found:

唔該 (mh goi) is used when someone does something for you that is expected or requested. For example, when you ask someone to hand you a tissue, you would say 唔該 "mh goi" to acknowledge he/she has helped you.

多謝 (daw jeh) is pretty much used all other times.

So getting back to that home cooked dinner at my future mother-in-law's house: I basically told her that it was expected of her to cook for me whenever I came over for dinner.

Talk about trying to be polite.