I speak Mandarin and cantonese, used to live in shanghai for 2 years and watch lots of taiwanese TV show. They are just so different from each other not only in accents. Each one has some particular words which make no sense at all in others. But the differece in accent is most important.

it's hard to know shanghainess,it pronounces quite different from pure madarin....also cantonese ..the same...but the three has some in common..taiwanese is more like cantonese than madarin... all the chinese recieve education shall learn madarin in order to get communicated more fluently.....

Mandarin of Chinese is Putong hua.普通话.Shanghaiese is a locality Chinese.Both use the same Chinese characters.But the pronunciation is have many differences.China is too large.So it has too many locality languages.Most of them are Chinese.Few of them are other languages.For example:Mongolian.I don't kown how to say Shanghaiese.But I can say ChangShaese.(ChangSha is the capital city of Hunan province)."Have no sth"in Chinese is 没xxx,the pinyin is "mei xxx"But in Changshaese we read it "mao ".If you have any questions,you can tell me. And pleaese tell me are there any mistakes is my anwser?

many differences....it's hard to distinguish them in a few sentences

taiwanese, shanghainese and cantonese is different dialcets in china. they have different pronounciation and own special charaters. like in cantonese the character "抦" bing3 it means as the charater 打da3 in mandarine. it means beat.(^.^)you can surf this website http://www.520hai.com/yueyu.htm . ther is many different dialects learning course on it.