Learn Hardcore Russian: Купи в магазине какой‑нибудь свежий хлеб к ужину. - Buy some fresh bread in the store for dinner.

Купи – informal singular (“you” = ты), used with friends, family, children, etc.
Купите – either polite singular or plural (“you” = вы), used with strangers, older people, groups, or in polite speech.

The rest of the sentence stays the same:

Купи в магазине какой‑нибудь свежий хлеб к ужину. (to a friend)
Купите в магазине какой‑нибудь свежий хлеб к ужину. (to a colleague you address as вы, or to several people)

Why is it в магазине and not в магазин or из магазина?

These three combinations answer different questions:

в магазине – “in the store / at the store”
- в
  - предложный падеж (prepositional case)
- Tells where the action happens: Купи (где?) в магазине. – Buy it in the store.
в магазин – “into the store / to the store”
- в
  - винительный падеж (accusative case) for direction
- Tells where to go: Иди в магазин. – Go to the store.
из магазина – “from the store”
- из
  - родительный падеж (genitive case)
- Tells from where something comes: Принеси хлеб из магазина. – Bring bread from the store.

In your sentence you want the place where you buy it, so в магазине is correct.

What exactly does какой‑нибудь mean here? How is it different from just свежий хлеб?

какой‑нибудь is an indefinite pronoun meaning something like “some / any (doesn’t matter which)”.

какой‑нибудь свежий хлеб – some fresh bread, any fresh bread is fine, the exact type is unimportant.
свежий хлеб (without какой‑нибудь) – fresh bread, but it can sound a bit more definite/expected, like “the fresh bread we usually buy” or simply a more neutral instruction.

So какой‑нибудь adds a nuance of indifference to the specific kind: the speaker doesn’t care which fresh bread you pick.

How is какой‑нибудь different from какой‑то?

Both are indefinite and can often both translate as “some”:

какой‑нибудь – more neutral “any kind / whichever”, often with a nuance of randomness or “it really doesn’t matter”.
какой‑то – “some kind of / some sort of”, often with a nuance of vagueness or slight uncertainty, sometimes even mild suspicion (“some weird bread”).

In your sentence, какой‑нибудь свежий хлеб is very natural and neutral:

Just any fresh bread, don’t overthink it.
Какой‑то свежий хлеб would also be possible, but can sound a bit more like “some (unspecified) fresh bread”, a bit less neutral.

What case is хлеб in, and why doesn’t it change its form?

хлеб is the direct object of Купи, so it is in the accusative case.

However, for masculine inanimate nouns like хлеб, the accusative form is identical to the nominative:

Nominative (subject): Хлеб вкусный. – The bread is tasty.
Accusative (object): Купи хлеб. – Buy (some) bread.

So the form хлеб looks the same, even though the grammatical case is different.

What does к ужину mean, and why do we use к here?

к ужину literally means “by dinner / for dinner (time)”.

к
- дательный падеж (dative case) (ужин → ужину)
к here expresses a time by which something should be ready:
- Сделай это к вечеру. – Do it by evening.
- Купи хлеб к ужину. – Buy bread so that by dinnertime we have it.

So the focus is on the deadline: the bread should already be there when dinner happens.

Can we say на ужин instead of к ужину? What is the difference?

Yes, both к ужину and на ужин can be used, but they have different typical nuances:

к ужину – by dinner (time)
- focuses on when it must be ready: by the time dinner starts.
- In your sentence, it emphasizes the deadline.
на ужин – for dinner (as part of the meal)
- focuses more on purpose: what the bread is for, what you will eat at dinner.
- Example: Приготовь что‑нибудь вкусное на ужин. – Cook something tasty for dinner (as the dinner meal).

In everyday speech you might hear either, but:

Купи хлеб к ужину. – Make sure we have bread by dinner time.
Купи хлеб на ужин. – Buy bread to have it as part of dinner (what we’ll eat).

Often the difference is subtle, and context decides which sounds more natural.

Why does свежий come before хлеб? Can it come after the noun?

In modern standard Russian, adjectives almost always come before the noun:

свежий хлеб – fresh bread

Putting the adjective after the noun (хлеб свежий) is possible, but it usually changes the feel and often the structure of the sentence:

Хлеб свежий. – The bread is fresh. (a whole sentence: “Bread is fresh.”)

Inside a noun phrase like yours (as an object), свежий хлеб is the normal order.
So in your sentence you should say какой‑нибудь свежий хлеб, not какой‑нибудь хлеб свежий.

Why is the verb perfective (Купи) and not Покупай? What would Покупай в магазине… mean?

Купи comes from купить (perfective), while Покупай comes from покупать (imperfective).

Купи – do one complete action: “Buy it (once, get it done).”
Покупай – do the action regularly / repeatedly / as a process:
- Покупай в магазине свежий хлеб к ужину. – Buy fresh bread in the store for dinner (as a habit / every time).

So in your original sentence, the speaker wants the person to buy bread this specific time, so the perfective imperative Купи is appropriate.

Why don’t we say ты in the sentence? Could we say Ты купи в магазине…?

Russian imperatives normally omit the subject pronoun. The person you are speaking to is understood from the verb form:

Купи в магазине… – “(You) buy it in the store…”

Adding ты is grammatically possible but adds extra emphasis, often emotional:

Ты купи в магазине какой‑нибудь свежий хлеб к ужину.

This can sound like:

you’re contrasting this person with someone else (“you buy it, not someone else”), or
you’re a bit insistent / annoyed (“Come on, you go and buy it already”).

So the neutral, normal command is without ты: Купи в магазине какой‑нибудь свежий хлеб к ужину.