Z kim idziesz do kina?

Because the preposition z meaning with requires the instrumental case in Polish.
The pronoun kto changes by case:

• nominative: kto = who
• instrumental: kim = with whom / by whom

So:

• kto = who
• z kim = with who(m)

That is why z kto is ungrammatical.

Kim is the instrumental singular form of kto.

In this sentence, the structure is:

• z
  • instrumental
• z kim = with whom

This is a very common pattern in Polish:

• z mamą = with mom
• z kolegą = with a male friend
• z Anną = with Anna
• z kim = with whom

Idziesz is the 2nd person singular form of the verb iść.

So:

• iść = to go
• idę = I go / I’m going
• idziesz = you go / you’re going
• idzie = he/she/it goes

Since the sentence is asking you, Polish uses idziesz.

Also, Polish usually does not need the pronoun ty (you), because the verb ending already shows the person.

Because Polish often drops subject pronouns when they are obvious from the verb form.

• idziesz already means you are going
• so Ty z kim idziesz do kina? is possible, but it sounds more emphatic

Compare:

• Z kim idziesz do kina? = neutral, normal
• Ty z kim idziesz do kina? = more like And you, who are you going with? or adds emphasis/contrast

Because the preposition do requires the genitive case.

The noun is:

• nominative: kino = cinema
• genitive: kina

So:

• do kina = to the cinema

This is a very common pattern:

• do domu = to the house / home
• do szkoły = to school
• do sklepu = to the shop
• do kina = to the cinema

Usually it means going to the cinema in the normal English sense, which can imply going there to watch a film.

Polish commonly says:

• iść do kina = to go to the cinema
• iść na film = to go to a film / to see a movie

So do kina focuses on the destination (the cinema), while na film focuses more on the event or what you’re going to watch.

Grammatically, idziesz is present tense. But in Polish, just like in English, the present tense can be used for a planned future action.

So Z kim idziesz do kina? can mean:

• Who are you going to the cinema with?
• Who are you going with to the cinema?

It often refers to a plan that is happening soon or is already arranged.

Yes, Polish word order is fairly flexible, but Z kim idziesz do kina? is the most natural neutral order.

Possible variations include:

• Z kim idziesz do kina? = neutral, standard
• Do kina z kim idziesz? = marked, unusual, often contrastive
• Idziesz do kina z kim? = possible in speech, but less standard as a neutral question

For a learner, the safest default is:

• Z kim idziesz do kina?

Here z means with.

Be careful, because z can also mean from, depending on context and the case used.

For example:

• z kimś = with someone
• z Warszawy = from Warsaw

In your sentence, because z is followed by kim in the instrumental case, the meaning is clearly with.

Literally, iść is a verb of motion that originally relates to going on foot in one direction. But in everyday Polish, iść do kina is a completely normal way to say go to the cinema, even if the exact means of transport is not the point.

This is one of those places where Polish and English do not match perfectly word-for-word.

Useful comparison:

• iść = to go, to be going, usually one-direction movement
• chodzić = to go regularly / to walk around / repeated movement

So:

• Idziesz do kina? = Are you going to the cinema?
• Chodzisz do kina często? = Do you go to the cinema often?

Yes. That is the most literal translation:

• z kim = with whom
• idziesz = are you going
• do kina = to the cinema

In natural modern English, people usually say:

• Who are you going to the cinema with?

But the Polish structure is closer to the more formal English With whom are you going to the cinema?

By itself, it is neutral in tone, but because it uses idziesz (2nd person singular), it is addressed to one person in a familiar singular you form.

Polish does not mark formality here through a different you the way some languages do inside the verb alone; for formal speech, Polish often uses Pan/Pani plus a 3rd person verb.

For example:

• Z kim idziesz do kina? = informal / familiar singular
• Z kim Pan idzie do kina? = formal to a man
• Z kim Pani idzie do kina? = formal to a woman

So your sentence is the normal version for talking to a friend, family member, classmate, etc.

A simple answer often starts with Z...

Examples:

• Z kolegą. = With a male friend.
• Z koleżanką. = With a female friend.
• Z moją siostrą. = With my sister.
• Z Anią. = With Ania.

Or as a full sentence:

• Idę do kina z kolegą. = I’m going to the cinema with a male friend.
• Idę z Anią. = I’m going with Ania.

Notice that after z, the noun also stays in the instrumental case.