What is the difference between Cantonese and Mandarin?
Big difference. Both are spoken languages. There is where the similarity ends. Yet the differences are so huge that the analogy is like comparing Spanish to Italian, in spoken forms. For a Chinese, if he only understands Cantonese, there is no way he can understand Mandarin, and vice versa. It is easier for a person who knows one of the two languages to learn the other, as both are the same in written forms, than those who do not know at all.
Cantonese is merely a dialect. which is commonly used in Hong Kong, Southern part of China and the overseas Chinese communities in North America, Australia, Western Europe and probably in South America as well. Whereas Mandarin is the China national language. As a matter of fact, Mandarin in Chinese is 国语, which literally means language of a nation. With Mandarin one can communicate with over 90% of the Chinese. Further, like myself, many of the Cantonese speakers can speak or understand Mandarin. So, with no disrespect to Cantonese speakers, as I am one of them, unless one wishes to communicate with a diminishing population of “Cantonese only” speaking persons, otherwise, there is little incentive to learn Cantonese instead of Mandarin.
Having said that, I have been approached by friends on the language social nets, such as italki and LiveMocha, to teach them Cantonese. They learn Cantonese mainly because they want to understand the Cantonese in Hong Kong TV soap operas or the songs sung by Hong Kong pop singers. Interestingly, one friend told me he wanted to do it because he wanted to know what his Hong Kong colleagues spoke behind his back.
How to learn Cantonese?
To tell the truth, I have never taught anyone Cantonese because I think, as a dialect, it is unteachable. There are no rules for speaking Cantonese. Further, tools for learning Cantonese are difficult to find. The best way to learn it is through watching Cantonese soap operas or listening to Cantonese songs or talking to Cantonese speakers. Whereas for learning Mandarin there is Pinyin, or romanizaton (alphabetization) of the language, a system which foreigners are more accustomed to. Besides there are vast free resources in the market, and more native speakers, to help one learn Mandarin.
Is it difficult to learn Cantonese?
I am a native Cantonese speaker so Cantonese presents no difficulties to me. In Hong Kong, which is a predominately Cantonese speaking place, I often see Mormon preachers who can speak perfect Cantonese. Most of them were not born in nor are they resident of Hong Kong. However, as they are required to preach to people in the street in Cantonese, they have a lot opportunities to practice the language. On the other hand there are a lot of expatriates who have worked or lived in Hong Kong for many many years, but they can only speak literally a handful (less than ten words) of Cantonese, let alone Mandarin. As they have secretaries or employees who help them in case they need to communicate in Cantonese or Chinese. Besides, Hong Kong is basically a bi-lingual (Chinese and English) city. One can get around Hong Kong with ease knowing only English. Whether it is easy or not it would be a matter of how often you practise the language. As I said before, learning Cantonese is more difficult than Mandarin. However, after one is able to speak Mandarin, it would then be easy to pick up Cantonese.
Is Mainland Mandarin different from Taiwan Mandarin?
Basically no. Only a slight difference in accent. The analogy is like between American English and British English.
What is the difference between traditional and simplified Chinese?
In a nutshell, as the names suggested one is simplified and the other is not. Other differences are the traditional Chinese is more calligraphic and simplified Chinese is romanitized and the former is used in Taiwan and Hong Kong, while the latter Mainland China. Needless to say the differences are not as simple as that. The Taiwanese are staunch supporters of the traditional Chinese and the Mainland the simplified. If I say the wrong thing I could get into trouble. That is why I do not want to say more.