W supermarkecie kupuję nabiał, bo w domu nie ma już jogurtu.

Because the preposition w meaning in requires the locative case when it describes location.

• supermarket is the dictionary form (nominative)
• after w, it becomes supermarkecie

So:

• supermarket = supermarket
• w supermarkecie = in the supermarket

This is a very common pattern in Polish:

• w domu = in the house / at home
• w sklepie = in the shop
• w banku = in the bank

Polish often drops subject pronouns when the verb already makes the subject clear.

Here, kupuję means I buy / I am buying, so ja is not necessary.

• kupuję = I buy / I’m buying
• ja kupuję = I buy / I’m buying, but with extra emphasis

So the sentence does not need ja unless you want contrast or emphasis, for example:

• Ja kupuję nabiał, a on kupuje chleb. = I’m buying dairy, and he is buying bread.

Kupuję is the 1st person singular present tense of kupować.

It can mean:

• I buy
• I am buying

Polish present tense often covers both the simple present and the present continuous in English. The exact meaning depends on context.

Also, kupować is imperfective, so it suggests:

• an ongoing action,
• a repeated action,
• or a general/habitual activity.

In this sentence, it most naturally means something like:

• I’m buying dairy at the supermarket because there’s no more yogurt at home or
• I buy dairy at the supermarket because there’s no more yogurt at home

Nabiał is a collective noun meaning dairy products.

It works a bit like English words such as:

• furniture
• food
• luggage

So instead of listing specific items, Polish can use nabiał to mean dairy in general:

• milk products
• yogurt
• cheese
• cream, etc.

That is why kupuję nabiał means I buy dairy / dairy products.

Bo means because and is very common in everyday spoken and written Polish.

In this sentence:

• bo w domu nie ma już jogurtu = because there is no more yogurt at home

You could also use ponieważ, which is more formal or bookish:

• W supermarkecie kupuję nabiał, ponieważ w domu nie ma już jogurtu.

Both are correct, but bo sounds more natural in ordinary conversation.

Because w domu means in the house / at home, so it describes location, not movement.

• w domu = in the house / at home
• do domu = to the house / homeward

In this sentence, the idea is that the yogurt is missing at home, so Polish uses w domu.

Compare:

• Jestem w domu. = I am at home.
• Idę do domu. = I am going home.

Nie ma is the normal way to say there is not / there isn’t / there are not in many contexts.

Here:

• w domu nie ma już jogurtu literally means something like at home there is no more yogurt

The verb is from mieć (to have), but in the expression nie ma it often functions like there isn’t / there are not.

This is a very common Polish structure:

• Nie ma chleba. = There is no bread.
• Nie ma czasu. = There is no time.
• W lodówce nie ma mleka. = There is no milk in the fridge.

Because after nie ma, Polish normally uses the genitive case.

The basic form is:

• jogurt = yogurt

But after nie ma, it becomes:

• jogurtu

So:

• Jest jogurt. = There is yogurt.
• Nie ma jogurtu. = There is no yogurt.

This is an important pattern to learn:

• jest + nominative
• nie ma + genitive

Other examples:

• Jest ser. = There is cheese.
• Nie ma sera. = There is no cheese.

Już often means already, but in negative sentences it very often corresponds to English any more / any longer.

So:

• nie ma już jogurtu = there is no more yogurt / there isn’t any yogurt left anymore

Without już, the sentence would simply state absence:

• nie ma jogurtu = there is no yogurt

With już, it adds the idea that there used to be some, but now it is gone.

Yes. Polish word order is more flexible than English because endings show grammatical relationships.

The sentence:

• W supermarkecie kupuję nabiał, bo w domu nie ma już jogurtu.

is natural and neutral.

But other orders are possible, for example:

• Kupuję nabiał w supermarkecie, bo w domu nie ma już jogurtu.
• Bo w domu nie ma już jogurtu, kupuję nabiał w supermarkecie.

These versions may sound slightly different in emphasis, but they are still understandable.

In general:

• earlier position often gives more emphasis or sets the scene
• later position can sound more neutral depending on context

It can mean both, and in many everyday sentences it is best translated as at home.

So here:

• w domu nie ma już jogurtu

is most naturally understood as:

• there is no more yogurt at home

Polish often uses w domu where English prefers at home rather than in the house. The exact English translation depends on context, but the Polish phrase itself is completely normal.