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The instrumental case is used to indicate the instrument/object with which an action or state of being is performed. For instance, when you go to work "by car", car is instrumental because it's the 'instrument' used to perform the action.

I've seen the following sentence, however, and can't make sense of why "brother" is instrumental:

"I go for a walk with my brother./Idę na spacer z moim bratem."

The verb iść already implies that the subject goes on foot (and bolstered by "for a walk" anyway), however, I don't understand either grammatically or morally, how is "brother" an instrument with which the subject goes on a walk. It seems to make no sense, "brother" does not help the person go on a walk, they're just an accompanying element, the means with which the subject goes on a walk is already implied in the verb itself. So, to me, I fail to see why is the instrumental case used in this sentence.

Can anyone clarify?

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    When I was learning English, I was baffled as to why people were saying "I eat with a spoon". What does it even mean? "Me and my spoon, we're eating?"
    – Quassnoi
    Sep 25 at 21:27
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I don't understand either grammatically or morally, how is "brother" an instrument with which the subject goes on a walk.

You are right, brother is not an instrument here. "I go with my brother" — this type of relation is called comitative semantic relation.

Morphologically, it behaves like Instrumental case, but functionally it serves a different purpose. Many IE languages do not differentiate the two (or do only via a preposition), hence the misunderstanding between the two:

The comitative case is often conflated or confused with other similar cases, especially the instrumental case and the associative case. — Wikipedia

Quite often, older languages had more noun cases which later merged. There are historic reasons for that, and these reasons are not always "logical" or "moral". One should simply memorize which (maybe unrelated) functions are represented by which noun case.

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    And note that in English (which has almost no surviving case system) the same preposition (with) is used for both instrumental and comitative senses.
    – Colin Fine
    Sep 25 at 14:48
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    So you say that "with my brother" is actually originally commutative case that, in Polish, has merged with instrumental?
    – FMB
    Sep 25 at 23:49
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    @FMB, exactly so.
    – bytebuster
    Sep 26 at 0:18
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    Is there any evidence that Polish or any of its ancestor languages actually has a comitative case? My understanding is that even PIE would use the instrumental case to express a comitative meaning. Some languages may have a comitative case - I don't believe any Indo-European languages are among them. Sep 27 at 1:50
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    @FMB No, it’s not originally a comitative case necessarily. A case is a morphological thing, a specific form of a word made (most commonly) by adding a specific ending. Cases are pure form, and they get used for lots of different things in different languages. The example here is a comitative semantic relation – that is, a way to express the notion that things go together or are ‘with’ each other. An actual comitative case is one way to express that, but it’s not how Polish or English do it – they both do it by using a preposition (z and with), in Polish with the instrumental case. Sep 28 at 22:14
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The preposition z meaning 'with' takes the instrumental case, is all. E.g.

Mieszkam w domu z ogrodem

You say

The instrumental case is used to indicate the instrument/object with which an action or state of being is performed.

...which is true, but that's not the only use of the instrumental.

  1. Of tools, instruments, and modes of travel
  2. After the prepositions z ('with'), przed, nad, pod
  3. For the objects of certain verbs like interesować się, zostać
  4. In some expressions of time, e.g. wieczorem
  5. With relations, nationalities, professions, e.g. Jestem nauczycielem

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