How does L1 affect L2 learning?

edit

Could you be a little more specific with your question? I am not sure exactly how to answer, but will try my best. There are several theories out there that might answer your question. A lot are based on whether you accept the idea of Universal Grammar (UG). (Check out wikipedia for a pretty good description). A very popular theory in second language acquisition is the Full Transfer/Full Access hypothesis by Sprouse and Schwartz. Traditionally, it's been thought that when we get language input as a baby this activates the parts of universal grammar that are relevant to our L1. Some say this means that our L1 grammar 'replaces' the universal grammar which is why it is then harder to learn an L2 because we can only process it through the L1 grammar perspective rather than the universal grammar perspective we had as babies. In L2 acquisition this is talked about as L1 interference. However, Sprouse and Schwartz claim that in learning an L2 all the applicable grammar from our L1 transfers and we still have access to the universal grammar. This would make it possible to reach near native-like fluency. Others who support the critical period hypothesis would say we lose access to the universal grammar after a certain age of adolescence. Therefore, learning an L2 with native-like proficiency is difficult or nearly impossible. Under this critical period hypothesis it follows that if the L2 shares many of the same characteristics of the L1 grammar then it should be slightly easier to learn.