Купи в магазине чего-нибудь к чаю.

Купи is the perfective imperative of купить. It means buy it / go and buy it, with focus on the result: the speaker wants the action completed.

By contrast, покупай is the imperfective imperative of покупать. That form can sound more like:

• be buying
• go ahead and buy
• buy regularly
• or a more process-oriented instruction

In this sentence, the speaker most likely means a one-time concrete action, so купи is the natural choice.

Because в can mean either:

• in / inside a place → usually takes the prepositional
• into a place → usually takes the accusative

Here, в магазине means in/at the store, so it answers the question where? and uses the prepositional:

• магазин → dictionary form
• в магазине → in the store / at the store

If it meant movement into the store, you would expect something like в магазин instead.

This is one of the most common questions in sentences like this.

Что-нибудь is the nominative/accusative form meaning something / anything.

Чего-нибудь is the genitive form of the same pronoun, and in sentences like this it often has a partitive feel: something of some kind, something or other, often especially with food or drink.

So:

• Купи что-нибудь = Buy something
• Купи чего-нибудь = Buy something / buy a little something / get something or other

The version with чего-нибудь often sounds a bit more natural and idiomatic when talking about buying snacks, food, drinks, or unspecified items.

It does not necessarily mean a different translation in simple English, but the Russian nuance is slightly more indefinite and colloquial.

It is both grammatical and very idiomatic.

Russian often uses the genitive after verbs like buy, bring, pour, eat, drink, especially when the thing is:

• indefinite
• uncounted
• partitive in meaning
• or just vaguely referred to

That is why forms like these are common:

• купи хлеба = buy some bread
• выпей чаю = drink some tea
• принеси воды = bring some water

So чего-нибудь fits that same pattern. It is not random.

That said, купи что-нибудь is also possible and correct. The sentence with чего-нибудь just sounds more natural in this everyday context.

Literally, к means something like toward / for, and it takes the dative case:

• чай
• к чаю

But in this expression, к чаю means something like:

• for tea
• to go with tea
• for tea time
• something to have with tea

So the speaker is not asking you to buy tea itself. They mean something like:

• cookies
• cake
• candy
• pastries
• something snack-like or sweet

This is a very common Russian expression.

Not exactly in all contexts.

In English, tea can mean:

• the drink
• or, in some varieties of English, an evening meal

In Russian, к чаю usually means to go with tea as a drink. It often suggests a sweet or snack served alongside tea.

So here it means more like:

• something for tea
• something to have with tea
• something to serve with tea

not something for dinner.

Because the preposition к requires the dative case.

So:

• чай → nominative
• к чаю → dative after к

This is simply a fixed grammar rule:

• к кому? к чему? = toward whom? toward what? / for what?

In this sentence, you do not need to think of physical movement. With к чаю, it is an idiomatic use meaning for tea / with tea.

Yes, absolutely.

That version is very natural and common.

The difference is mainly nuance:

• что-нибудь = something
• чего-нибудь = something or other / a little something, often more idiomatic with food-like purchases

For many learners, it is enough to remember:

• both are possible
• чего-нибудь often sounds especially natural in everyday speech when asking someone to buy an unspecified edible item

Russian does not need to name the item explicitly here. The pronoun чего-нибудь stands on its own and means something / anything.

But in context, native speakers usually understand it as something like:

• чего-нибудь вкусного = something tasty
• чего-нибудь сладкого = something sweet
• something to snack on

So the sentence leaves the exact item unspecified on purpose.

Russian word order is flexible, but this order sounds natural.

A rough breakdown is:

• Купи = the action
• в магазине = where to do it
• чего-нибудь = what to buy
• к чаю = what it is for

This order feels like a normal everyday request.

You could change the order for emphasis, for example:

• Купи чего-нибудь к чаю в магазине
• В магазине купи чего-нибудь к чаю

These are possible, but they shift the focus slightly. The original sentence is a neutral, natural version.

It is a normal informal singular command.

Купи is used when speaking to:

• one person
• someone you address with ты
• a friend, family member, child, partner, etc.

It is not rude by itself. Tone and context matter.

If you wanted a more polite version for вы, you would say:

• Купите в магазине чего-нибудь к чаю.

If you wanted to soften it further, you might say:

• Купи, пожалуйста, в магазине чего-нибудь к чаю.

Yes. Very strongly.

Because of к чаю, many Russian speakers will naturally think of:

• cookies
• cake
• chocolate
• pastries
• candy
• something sweet or snack-like

So even though the sentence literally says only something for tea, the cultural implication is often something nice to eat with tea.