Hi Jon, "im" answers the question "Where is it?" It tells you that the object is in a certain place. This is the Dativ case. "in den" indicates that you're putting the object in a different place (in your example: from outside the fridge into the fridge). This is the Akkusativ case. Another example is: Ich bin im Park = I am in the park Ich gehe in den Park = I am going to the park

Because "in" is one of the many prepositions in German that takes the dative ("in dem" shortened to "im" for a masculine or neuter noun) when you are talking about position ("he is standing in the fridge" in this case) and the accusative ("in den") when you are talking about movement ("since when have you been putting oil in(to) the fridge" in this case) You will probably find lots of more detailed explanations and lists of which cases each German preposition can take if you search for "German preposition cases" or something similar.

"In" is one of the prepositions that can be used with dative or accusative, depending on whether you're talking about a location (dative, first example) or a direction or motion (accusative, second example). See, for example: http://german.about.com/library/weekly/aa052101a.htm