Prepositions are important words in sentences as they show the relation between the words. Common prepositions in English include: with, in, for, to etc. One major difference between English and German grammar is that each preposition in the German language is associated with a specific case. Here we are looking at a specific subset of prepositions called two-way prepositions which can be used with either the accusative or dative case. The following is the complete list of nine two-way prepositions:

in (into, in)                                               über (over, above)

an (to, onto, on)                                      unter (under, below)

auf (onto, on)                                           vor (in front of)

hinter (behind)                                        zwischen (between)

neben (next to) 

1. The accusative is used if there is clearly motion from point A to B. Generally verbs of motion fall into this category. Commonly used verbs include to go (gehen), to drive (fahren), to fly (fliegen), to fall (fallen) etc.

I’m going into the car.                                  Ich gehe in das Auto.

I am driving into the city.                           Ich fahre in die­ Stadt.

2. The dative is used if there is no motion, or motion occurs in a circular pattern or restricted area wherein you come back to your starting point i.e. pacing back and forth, swimming laps, jumping on the spot.

I am in the car.                                              Ich bin in dem Auto.

I am swimming in the pool.                     Ich schwimme in dem Becken.

3. Specific verbs are used in German to clearly show in what position something is placed. For example, if a book is placed flat on a table one would use a different verb than if it was placed upright in a shelf.

Accusative                                     Dative

stellen  (to place upright)                       stehen  (to be placed upright)

legen    (to place flat)                                liegen   (to be placed flat)

setzen   (to place sitting up)                  sitzen   (to be seated)

stecken (to stick somewhere)               stecken (to be stuck somewhere)

hängen  (to hang up)                                hängen (to be hanging)

Examples:

I am placing the cup on the table.         Ich stelle die Tasse auf den­­­ Tisch.

The cup is on the table.                             Die Tasse steht auf dem Tisch.

4. Wo is always used in the dative case as it shows where something is located whereas wohin indicates motion and is therefore used with the accusative case.

Where are you?                                            Wo bist du?

Where are you going (to)?                       Wohin gehst du?

5. Certain prepositions may be contracted with definite articles in both the accusative and dative. While these contractions are not mandatory, they are certainly common in both speech and writing and should be memorized. Some examples follow:

Accusative                                    Dative

auf + das = aufs                                        in + dem = im

in + das = ins                                             an + dem = am