Why do we use the dative here?

Question

I came across this sentence:

In meiner Freizeit lese ich Bücher.

Could you please explain to me why this sentence is in dative. In other words, why can't I say:

In meine Freizeit lese ich Bücher.

Answer 1

Sentences are never in any case! A grammatical case is not a property of a sentence. It is a property of a noun phrase. (More about this below)

But first let's talk about the case of "meiner":

The word "meiner" is a possessive pronoun that is in the role of a determiner in this sentence. It determines the noun "Freizeit". Therefore "meiner Freizeit" is a noun phrase (also called "nominal phrase"). Noun phrases are the building blocks that carry a grammatical case. This case can depend on one of three things:

1. The sentences main verb
   German sentences and clauses can contain many verbs, but per clause there is only one main verb. All other verbs are modal verbs or auxiliary verbs (»Ich werde bald tanzen können«: werde = auxiliary verb indicating the tense (Futur I), tanzen = main verb, können = modal verb indicating the possibility).
   Each verb has its individual list of attendants that it needs. One of them, that every verb needs, is the subject, that always is in nominative case. The other attendants, that are ruled by the main verb can be:

   • equating nominative (this does not count as an object, but as part of the predicate)

     Marillen sind Aprikosen.
     Apricots are apricots. (Marille is the synonym of Aprikose used in Austria.)

   • genitive object

     Wir gedenken der Toten.
     We commemorate the dead.

   • dative object

     Das gehört dem Chef.
     That belongs to the boss.

   • accusative object

     Sie sehen den Mann.
     They see the man.

   • prepositional object (prepositional objects do not carry a case themselves, but they contain an inner object that is in a certain case)

     Ilse denkt an den Urlaub.
     Ilse thinks about her vacation.

2. Being a genitive attribute (an inner noun phrase in genitive case inside an outer noun phrase in any case)

   • left genitive attribute (outer noun phrase is in nominative case)

     Ihr Ziel war des Mannes rote Nase.
     Her target was the man's red nose.

   • right genitive attribute (outer noun phrase is in nominative case)

     Ihr Ziel war die rote Nase des Mannes.
     Her target was the red nose of the man.

   • right genitive attribute (outer noun phrase is in genitive case)

     Wir gedenken der Toten des schrecklichen Unglücks.
     We commemorate the dead of this terrible disaster.

   • right genitive attribute (the outer noun phrase is a noun phrase in accusative case which itself is the inner noun phrase of a prepositional object in no case)

     Markus denkt an die Hochzeit seiner Tochter.
     Markus thinks of his daughter's wedding.

3. The preposition of a prepositional phrase
   Prepositional phrases can be prepositional objects ("Präpositionalobjekt"), as described above, but also adverbials ("adverbielle Bestimmung"). The inner structure of both is identical. What makes the difference between an adverbial and a prepositional object is how this phrase is used in the sentence.

   • some prepositions need their inner object to be in genitive case

     Das Geschäft ist trotz des Umbaus geöffnet.
     The store is open despite the renovation.

     Während des Sturms fiel der Strom aus.
     The power went out during the storm.

   • some other prepositions need the inner object to be in dative case

     Ernst konnte nach dem Unfall 3 Wochen lang nicht gehen.
     Ernst was unable to walk for 3 weeks after the accident.

     Martin ging zu dem Geschäft.
     Martin went to the store.

   • some need always accusative case

     Simon kämpft gegen den Titelverteidiger.
     Simon fights against the defending champion.

     Sandra watet durch den Fluss.
     Sandra wades through the river.

   • And there is another class of prepositions, that are called "Wechselpräpositionen" that take dative if its a movement at a location but accusative if its a movement towards a location

     • dative: at (inside) a location

       Klaus geht in der Halle.
       Klaus walks inside the hall.

     • accusative: towards a location

       Klaus geht in die Halle.
       Klaus walks into the hall.

       Wechselpräpositionen are relatively easy to understand when it's about locations, but they also work as temporal prepositions.

     • dative: at a fixed time

       Ich werde dich in unserem Urlaub verwöhnen.
       I will spoil you on our vacation.

     • accusative: temporal movement

       Ich musste den Arzttermin leider in unseren Urlaub verschieben.
       Unfortunately, I had to postpone the doctor's appointment to our vacation.

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In your sentence:

The phrase "meiner Freizeit" is inside a prepositional phrase that begins with the preposition "in" which is a Wechselpräposition and which therefore can demand either accusative or dative case. Here it is used as a temporal preposition, referring to a fixed time (there is no temporal movement), and therefore it needs dative case. And this case does not depend on the verb, because a phrase that is inside a prepositional phrase never "sees" the verb. It sees only the preposition.

The verb lesen demands the object that contains what is read in accusative case, and therefore this object (Bücher) is in accusative case. But the subject ("ich") is in nominative case. The adverbial "in meiner Freizeit" is a prepositional phrase in no case at all, but it has an inner object in dative case.

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The German and the English case system have a lot in common:

Cases are properties of noun phrases. A noun phrase can be a single noun (like "trees" in "I see trees") or a noun together with other words that determine or describe it (determiners and attributes). These other words are most often articles and adjectives, but sometimes also other kinds of words like numerals, participles or even other parts of speech. Even (inner) noun phrases can be attributes of an (outer) noun phrase. Also single pronouns count as noun phrases, although they do not contain any noun. Everything that is marked bold is a noun phrase in the next examples:

• Just a single noun:

  I see trees.

• A quantifier, an enumeration of adjectives and a noun:

  I see many tall, old and green trees.

• A numeral, a participle and a noun:

  I see two burning trees.

• An inner noun phrase and a noun. The inner noun phrase is "the angry forest owner's". It consists of an article (the), an adjective (angry) and an open compound noun (forest owner):

  I see the angry forest owner's trees.

• A single pronoun

  I see them.

And this all is true for German too:

Ich sehe Bäume.
Ich sehe viele große, alte und grüne Bäume.
Ich sehe zwei brennende Bäume.
Ich sehe die Bäume des wütenden Waldbesitzers.
Ich sehe sie.

In both languages you can have as many noun phrases as you want, and they can be in different individual cases:

He lives in his house because it belongs to him.

The three pronouns are in the three different cases that English has:

• He is in nominative case which sometimes is also called "subjective case".
• his house is a noun phrase in objective case, but it contains an inner noun phrase which is a single pronoun in possessive case ("his").
• him in objective case.

It is nonsensical to say that the whole sentence is in one of these cases.

Note, that in English also nouns are marked (with apostrophe and "s") when they are used in possessive case:

nominative: The man sleeps.
possessive: This is the man's hat.
objective: The hat belongs to the man.

The difference to German is, that German has not 3 but 4 cases, of which only one (nominative) directly matches with a case of the English case system.

Er beschuldigt den Mann mit dem blauen Hut des Mordes.
He accuses the man with the blue hat of murder.

• Er is in nominative case (the term "subjective case" is not used in German grammar, but still this noun phrase is the subject of the sentence)
• den Mann mit dem blauen Hut is a noun phrase in accusative case. The core is the noun "Mann", but it also contains an inner prepositional phrase that is an attribute of "Mann".
• mit dem blauen Hut is a prepositional phrase. It is a right attribute of the noun "Mann". Every prepositional phrase begins with a preposition (here: mit) and the rest is a noun phrase in a case determined by this preposition
• dem blauen Hut is a noun phrase in dative case inside a prepositional phrase inside an outer noun phrase
• des Mordes is a noun phrase in genitive case.

But besides what kinds of cases exist, there is also another difference between English and German: In English only pronouns and nouns (later only in possessive case) are marked according to the case in which they appear. But no other kind of word (part of speech) is affected by the case. So, while "green trees" in "Green trees are tall" and "I see green trees" appears in fact in two different cases in English, they still look identically, and so you don't need to care much about cases in English: Add an apostrophe and an s to a noun when you want to indicate the owner of something and use he/his/him (I/my/me; you/yours/your; etc.) correctly. Thats all you need to care about English cases.

But in German nouns are declined according to all four cases, and in addition also all words that are uses as determiners or attributes must also be declined according to the case of the noun to which they are connected.

Answer 2

The preposition in may either take accusative case or dative case but there's a difference in meaning.

• With dative case, it marks a location in space or time.
• With accusative case, it marks a direction or a transition.

And that's the reason why you can't use accusative here. You don't have a transition nor a direction with Freizeit. It all happens within that Freizeit.

In meiner Freizeit lese ich Bücher.

With times, accusative with in and the other Wechselpräpositionen is common when you talk about deadlines. But note this is all a matter of viewpoints and their specific verbs:

Die Fertigstellung des Projekts zieht sich bis in den August.

Das Projekt wird im August fertiggestellt.

The first sentence uses the verb sich ziehen which means the action Fertigstellung "elongates" into a later time frame. That's a transition.

The second sentence on the other hand uses the verb fertigstellen directly which describes a singular time rather than a longer period. So it can't be a transition.

This logic is by the way not limited to in or Wechselpräpositionen specifically. It also works with other prepositions.

Die Fertigstellung des Projekts zieht sich zum August hin.

That preposition zu always means a direction or transition. It always takes dative case nonetheless.