Hi Saleh, you are indeed discussing 'something' here. I think it's a feedback mechanism of earning one's trust and respect. But having been relating to italki for some time, nothing can surprise me now. Probably be a cultural issue or an individual issue there--to me, it's even better to say 'I don't need your help' or 'I don't like your help' than show nothing but indifference, but I don't care now. However, whenever I get help from others I definitely say or do something thankful--even it might sound wordy or seem inessential at times but that's the way we, most Asian people I believe, were brought up.

Hi ,Your chinese make us feel confused !

Xie xie is a pharase used as common as Thank you in english !

No matter when you feel apreciate ,you can say Xie Xie ,and ppl who heard this would feel as the same when you say thank you to ppl  in english .

if you want to emphasize the Xie Xie .u  can say " 真是太感谢了 Zhen Shi Tai Gan Xie le !" or "非常感谢 Fei Chang Gan Xie” which both means "thank you so much !"

Generally Speaking ,after you said "Xie Xie " ppl would answer "不用谢  bu yong xie " " 不客气  bu ke qi " " 没事“Mei shi " .

I would say 感激不尽(Gan3 Ji1 Bu2 Jin4)。

感激，means appreciate

不尽，means not ending at all.

the whole meaning will be: I appreciate very deeply without ending.

For me , Saying Thank you when you feel grateful to someone , or sorry when you make a mistake is a BIG BIG courage ! 

对不起　，　我知道我的汉语不很好，　但是我必须实践 (I must practice) 

Thank you all for trying to help answering my question :)