Herro?

But "dou" is quicksand for the beginner [Cantonese speaker]. Depending where you place it and how you pitch it, it can mean here, there, where, more, how much, to, from, able to accomplish, able to reach, everybody, and also. It can also mean gamble and knife. Really, in Cantonese the dou is out of hand.

-Daisann McLane, Slate Magazine

Learning a new language can be a tricky thing. Sometimes it comes and sometimes it stalls. Like today, when I was flipping channels and stopped on a soap opera because for whatever reason I understood everything everyone was saying. Then I changed channels and was lost again. It's something that just takes time, I guess.

My native tongue is Cantonese (though I've let that slip), but I came to China with only some basic knowledge of Mandarin. And yeah, both those things help, but maybe not as much as some friends back home think. Cantonese, for example, has 8 different tones; Mandarin has "only" 5. The extra tones serve to make Cantonese "choppier"; people tend not to slur their words since more precision is needed. So I've been making the transition, but I'm still at the point where I can understand more than I can speak. (And to add to my struggles, I'm now learning words in Mandarin that I never knew in Cantonese. Very confusing sometimes.)

For the record, there is no universal "Chinese" language; there are only a countless number of local dialects (like it is in India). Cantonese is most popular in the southern province of Guangdong (i.e. Canton) and Hong Kong, Shanghainese is only spoken in Shanghai, Xi'an-nese is popular around Xi'an, and so on. Mandarin became the default dialect only because the government decided it would be (probably because it's the native tongue of Beijingers). In 1955, the government even renamed Mandarin putonghua, literally the "normal language". Now pretty much everyone speaks it because they had to take it in school.

A couple related notes:

• If you're wondering why, if Mandarin is the dominant language, that there seem to be a disproportionate majority of Cantonese speakers in your Chinatown, it's because a vast majority of Chinese emigres used to come from Hong Kong. That's also why the West used to know the Chinese capital as Peking (the Cantonese pronunciation) instead of the now-standard Beijing (the putonghua pronunciation).
  

• For what it's worth, ketchup/catsup is a Cantonese word too (literally, tomato juice).  That's why nobody can quite figure out how to spell it.
  

• Xi'an is not the same as xian. The apostrophes are used to split words up: xi is "west", and an is "peace". Together, they're the name of that city where the terracotta warriors were found. Xian (one word) could mean a few different things, like "first" or "fresh".