Hi Salma26, hope you're well and doing well at school. well, col********s are pairs or groups of words that usually go together in a way that they sound natural and also meaningful .. for example we say "make an effort" not "do an effort" and we say "have an accident" not "make an accident "
well, I have enclosed a link below from which u can download an e-book about col********.
cheers
http://www.4shared.com/file/77639384...ns_in_use.html