What's the diference between nel/nello!?
a similar question was asked here: http://duolingo.com/#/comment/402125 One of the members was kind enough to provide this resource as a response: http://www.zanichellibenvenuti.it/wordpress/?p=1365
The site appears all in Italian, but this particular page has a very nice table in the middle showing the forms for the common Italian prepositions. Very handy reference.