In English, prepositions, such as "to," "for," "with" and "against," are followed by their objects. In the prepositional phrase, "to the train station," "train station" is the object of the preposition "to."

Each German preposition calls for a specific case for its object. Most common prepositions take either dative or accusative.

The Preposition "zu" and its Contractions

The preposition zu takes the dative and means "to" (a certain place). The contraction for zu dem is zum. Dem is the dative singular form for the definite article in masculine and neuter:
 

MASCULINE	NEUTER
zum Bahnhof	zum Hotel
to the train station	to the hotel

The contraction for zu and der is zur. Der is the dative singular form for the definite article in feminine:
 

FEMININE

zur Bank

to the bank

The Preposition "an" and its Contractions

The preposition an takes the dative to describe the position of something at or on the side of something. The contraction for an and dem is am.
 

Steigen Sie am Hauptbahnhof aus.

Get off at the main train station.

Before feminine singular nouns, its form is not contracted.
 

Steigen Sie an der Hauptwache aus.

Get off at the Central Guard House.