To express a definite time (when), duration (how long), or distance (how far) without a preposition, put the phrase into the accusative case. This is known as the adverbial accusative.
Because feminine, neuter, and plural nouns generally don't change their forms in the accusative, you only need to pay special attention to masculine nouns. They take the -en ending. For example, der Herbst becomes jeden Herbst, and der Tag becomes den ganzen Tag.
Time and Distance Combined
You can easily stack these accusative phrases in a single sentence. Below, notice how the masculine time phrase (jeder Herbst ➔ jeden Herbst) and the feminine distance phrase (eine weite Strecke) both sit in the accusative without needing any extra connecting prepositions.
Expressing Duration (How Long)
To emphasize that an action lasts for an entire period of time, pair den ganzen (the whole) with a masculine time noun.
Definite Time (When)
Just like duration, specific points in time use this exact same pattern. Words like jeder (every) and dieser (this) take the masculine accusative ending -en when modifying masculine time nouns like Winter or Monat.
Paths and Measurements
The adverbial accusative isn't just for time—it is also the standard way to express physical paths and exact measurements. "The whole way" (den ganzen Weg) and "every kilometer" (jeden Kilometer) follow the same masculine accusative rule.
Finally, when stating an exact distance with numbers (like 1,000 kilometers), the plural noun just drops right into the sentence. Plural nouns do not change their forms in the accusative.
