20.1.5 An = to, on, at (with Dative and Accusative)

  If using the preposition an with verbs of movement the declination has to be Accusative , if it is used with other verbs the Dative follows.
  Er stellt es an den Anfang. = He puts it at the start.
  Er trat an mich heran. = He approached me.
  Er hängte das Bild an die Wand. = He hung the painting on the wall.
  Er schickt den Brief an mich. = He sends the letter to me.
  Er wohnt an der Gutenbergstrasse. = He lives on Gutenberg street.
  Das Bild hängt an der Wand. = The painting hangs on the wall.

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