Präpositionen.
Small words, fixed cases.

A preposition links two parts of a sentence: a verb and a place, a noun and a time, an action and a reason. In English, the preposition itself rarely changes anything else around it. In German, every preposition decides the case of the noun phrase that follows it. Learn the preposition, and you already know whether der becomes den, dem, or des.

Some prepositions only take the Akkusativ

A small fixed group of prepositions always sends the following noun phrase into the Akkusativ, and you never have to read the context to be sure.

Preposition	Meaning	Example
durch	through	Ich gehe durchden Park
für	for	Das ist fürdich
ohne	without	Ich gehe ohneden Schlüssel
um	around / at (time)	Wir sitzen umden Tisch

The full list, with every example: These prepositions only speak Akkusativ.

Some prepositions only take the Dativ

A second fixed group works just as automatically, always sending the following noun phrase into the Dativ.

Preposition	Meaning	Example
mit	with	Ich gehe mitdem Freund
bei	at / with / near	Ich bin beider Arbeit
nach	after / to (a place)	Ich komme nachdem Essen
von	from / of / by	Das ist voneinem Freund

Two of the fixed Dativ prepositions deserve their own look → zu and aus both describe movement, to a place or out of a place, and a learner's instinct is often to expect the Akkusativ for movement. These two stay Dativ regardless.

The full list, with every example: These prepositions only speak Dativ. And the movement exception: zu. Feels like movement. But always Dativ.

Two-way prepositions → the verb is the signal

A third group can take either the Akkusativ or the Dativ, and the choice depends on what the sentence says, not on the preposition itself. Ask Wohin? Movement to a destination takes the Akkusativ. Ask Wo? A fixed location takes the Dativ. The verb usually points the way, but the question is what decides.

Preposition	Location (Dativ)	Direction (Akkusativ)
in	Ich bin inder Küche	Ich gehe indie Küche
an	Es hängt ander Wand	Ich hängees andie Wand
auf	Es liegt aufdem Tisch	Ich legees aufden Tisch

The full picture, with all nine two-way prepositions and the verb pairs that signal each case: Same preposition. The verb decides.

A few prepositions take the Genitiv

The smallest group sends the following noun phrase into the Genitiv. These mostly appear in writing and formal speech, where the Genitiv itself survives most strongly.

Preposition	Meaning	Example
während	during	Währenddes Kurses war ich krank
trotz	despite	Trotzdes Regens sind wir gegangen
wegen	because of	Wegender Arbeit bin ich müde

The full list, with the spoken-German alternative: These prepositions only speak Genitiv.

Prepositions can merge with articles

Several prepositions regularly contract with the definite article into a single word. These contractions are standard, not informal shortcuts, and the case underneath them does not change.

Preposition + article	Contraction
in + dem	im
an + dem	am
zu + dem	zum
bei + dem	beim
in + das	ins
an + das	ans