I'm sure you worked out that all these sentences contained little words which tell you where something is. These words are called Prepositions of Location.
In German these words affect the noun before which they stand. Therefore the nouns after the preposition of location have to be put into the Dative Case. This exercise lets you work out how the Dative case affects nouns.
Look at the sentences below and work out how the words "der", "die" and "das" as well as "ein", "eine" and "ein" change.
der Fußballplatz - Jens Jeremies ist auf dem Fußballplatz.
die Sporthalle - Herr Bullock unterrichtet in der Sporthalle.
das Schwimmbad - Ich schwimme gern im Schwimmbad.
ein Park - Ich spiele oft Tennis in einem Park.
eine Bäckerei - Ich kaufe Brötchen in einer Bäckerei.
ein Basketballfeld - Ich spiele nachmittags auf einem Basketballfeld.
Did you get all this right? Then take a note of how the articles change for future reference. For example for when you do Activity 3...