Hello Alexliu, Probably you have learned English at school ,usually the courses ,lessons and books read in class don't include such informal language or if then at a minimum rate. So if you happened later to read informal words anywhere else , hear them in movies or from friends you will realize that this word is a bit odd or weird . When instantly inquiring about its meaning, you will gradually learn a lot of informal words. However there are some dictionaries for slang and informal words that you might like to check: www.peevish.co.uk/slang/a.htm www.bu.edu/mfeldman/Slang

Hello Alex I guess only the practice give you the possibility to use them correctly...