Prepositional case

In the Skill 'Where' there is no actual explanation of how to form a prepositional case from a nomitive case. Aren't there some general rules for that (of course with some/a lot of exceptions) in stead of just learning a few examples now, without the rules behind them? If so, could they please be added to the 'Tips and notes'. Thanks!

June 21, 2016
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