'I have studied since 1999" is the present perfect tense. It means the action began in the past(in the example is 1999), and finish it now. However, "I have been studying since 1999" means the action began in 1999, and now you are still studying.

When using the word "since," it sounds more natural to use the present perfect progressive ("I have been studying..."). "Since" implies there is an action continuing into the present and would not be as compatible with the present perfect tense ("I have studied...") The following would be a little more accurate: 1. I have studied English (before). (completed, note: the term "since" is not used here) 2. I have been studying English since 1999. (continuing into the present) Source: myself, American English

我认为的区别是：前者是现在完成时，现在已没有这个动作了，而后者是现在完成进行时，动作从过去开始进行一直持续到现在还在继续。不知道这个答案有没有帮到你。

Hey... Why do you write with hieroglyphs,Huili? It is usefull material and I don't understand anything. Don't offend at me but this is the truth. :-)