Why Chinese is hard to learn.

Some chinese people over here in France were explaining some things about their language to me. It's really bizarre how each vowel can pronounced in either a monotone, a descending tone, an ascending tone or a down-and-up tone.

That and all the different ideograms.

Vietnamese has accent marks --diacritic-- for that.

It's not like in french where it changes the vowel entirely (e.g: e as uh -> é as eh -> è as err), but like you described.