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.

Nel = in + il Nello = in + lo LO is used instead of IL when a masculine noun starts with s+consonant or z. Example: Lo specchio (the mirror). Then "Nello specchio" means "in the mirror".