20.1.8 auf = on (with Dative and Accusative)

  If using the preposition auf with verbs of movement the declination has to be Accusative , if it is used with other verbs the Dative follows.  
  with Dative  
  Der Aschenbecher steht auf dem Tisch.
= The ash tray is on the table.
  with Accusative  
  Sie stellt den Aschenbecher auf den Tisch.
= She puts the ash tray on the table

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