W sklepie jest dziś przecena na jogurt i mleko.

Because w meaning in or inside usually takes the locative case in Polish when it describes location.

• sklep = shop / store
• w sklepie = in the shop / in the store

So here:

• W sklepie = In the store

If you saw w sklep, that would be incorrect in standard Polish for this meaning.

Przecena means a price reduction, discount, or markdown.

In this sentence, jest dziś przecena na jogurt i mleko means that yogurt and milk are currently being sold at reduced prices.

It is slightly different from some related words:

• przecena = a reduction in price
• promocja = a promotion / special offer
• wyprzedaż = a sale, often a larger clearance-type sale

So przecena na jogurt i mleko is basically a discount on yogurt and milk.

Because the verb agrees with przecena, not with jogurt i mleko.

The structure is:

• jest przecena = there is a discount
• na jogurt i mleko = on yogurt and milk

So grammatically, the main noun is przecena and it is singular, which is why Polish uses jest.

A rough structure in English would be:

• There is a discount on yogurt and milk.

Not:

• There are yogurt and milk.

Here na means on in the sense of on a product / for a product.

So:

• przecena na jogurt = a discount on yogurt
• przecena na mleko = a discount on milk

This is a very common Polish pattern:

• promocja na kawę = a promotion on coffee
• zniżka na bilety = a discount on tickets
• wyprzedaż na buty = a sale on shoes

After na in this meaning, Polish uses the accusative case.

Because in the accusative, these nouns happen to look the same as in the nominative.

• jogurt is a masculine inanimate noun, so its accusative singular is the same as the nominative:
  • jogurt → jogurt
• mleko is a neuter noun, and its accusative singular is also the same as the nominative:
  • mleko → mleko

So even though the case changes grammatically, the form does not change visibly here.

Dziś means today.

Yes, it can move around quite freely in Polish. The sentence:

• W sklepie jest dziś przecena na jogurt i mleko.

could also be:

• Dziś w sklepie jest przecena na jogurt i mleko.
• W sklepie dziś jest przecena na jogurt i mleko.

All of these are natural. The difference is mostly about emphasis or rhythm, not basic meaning.

In the original sentence, dziś sits in the middle and sounds quite natural.

Yes, very much so.

Polish often uses jest in this kind of existential sentence:

• jest przecena = there is a discount
• jest problem = there is a problem
• jest kawa = there is coffee

So W sklepie jest dziś przecena na jogurt i mleko is literally close to:

• In the store there is today a discount on yogurt and milk.

Natural English would usually be:

• There’s a discount on yogurt and milk in the store today.

Because Polish does not have articles like English a/an/the.

So sklep can mean:

• a shop
• the shop
• sometimes simply shop/store in a general sense

And przecena can mean:

• a discount
• the discount

The exact meaning depends on context.

In this sentence, English usually needs an article, so we translate it as something like:

• There is a discount on yogurt and milk in the store today.

Yes. In many everyday contexts, w sklepie can correspond to English in the store or at the store, depending on how natural the translation sounds.

Literally, w means in, but English often prefers at in some situations. For example:

• Jestem w sklepie. = I’m at the store.
• literally: I’m in the store.

In your sentence, both are possible depending on context:

• In the store, there’s a discount today...
• At the store, there’s a discount today...

Yes, it is natural.

A native speaker might also say:

• Dziś w sklepie jest przecena na jogurt i mleko.
• W sklepie jest dzisiaj przecena na jogurt i mleko.

Here are a couple of notes:

• dziś and dzisiaj both mean today
• word order is flexible, so several versions sound normal

The original sentence is perfectly fine and idiomatic.

They both mean today.

The difference is mainly stylistic:

• dziś = slightly shorter, a bit more compact
• dzisiaj = full form, also very common

In everyday Polish, both are natural. So you could say either:

• W sklepie jest dziś przecena na jogurt i mleko.
• W sklepie jest dzisiaj przecena na jogurt i mleko.

There is no important difference in meaning here.

A helpful breakdown is:

• W sklepie = in the store
• jest = there is / is
• dziś = today
• przecena = a discount / markdown
• na jogurt i mleko = on yogurt and milk

So a good English-style understanding is:

• There is a discount on yogurt and milk in the store today.

Even though the Polish word order is different from the most natural English order, the sentence structure is very common in Polish.