Вона виходить з магазину з важким пакетом, а в пакеті — молоко і хліб.

Because з can express different relationships depending on the noun form that follows it.

• з магазину = from the shop
  Here з shows movement away from somewhere, so магазин becomes магазину in the genitive case.

• з важким пакетом = with a heavy bag
  Here з means with, describing accompaniment or something someone is carrying. That requires the instrumental case, so важкий пакет becomes важким пакетом.

So the preposition is the same, but the meaning and case depend on the context.

Because after з meaning from, Ukrainian uses the genitive case.

• dictionary form: магазин
• after з meaning from: з магазину

This is very common with motion away from a place:

• з дому = from the house / from home
• з міста = from the city
• з роботи = from work

So виходить з магазину literally means goes out from the shop.

Because after з meaning with, Ukrainian usually uses the instrumental case.

So:

• важкий пакет = a heavy bag
• з важким пакетом = with a heavy bag

Both the adjective and the noun change:

• важкий → важким
• пакет → пакетом

This is the normal pattern for with in the sense of accompaniment or what someone has with them.

Because they are two different cases used for two different functions.

• з важким пакетом
  Instrumental case: with a heavy bag

• в пакеті
  Locative case: in the bag

So:

• пакет = base form
• пакетом = instrumental singular
• пакеті = locative singular

English usually keeps the noun unchanged and uses prepositions instead, but Ukrainian often changes the noun ending as well.

Because в can mean either into or in, and the case changes depending on the meaning.

• в пакет = into the bag
  This suggests movement into it, and usually takes the accusative
• в пакеті = in the bag
  This describes location, so it takes the locative

In this sentence, the milk and bread are already inside the bag, so в пакеті is correct.

Виходить is the 3rd person singular present tense of виходити.

So it can mean:

• she goes out
• she is going out
• she comes out / is coming out

The exact English translation depends on context. In this sentence, it describes an action happening now or as part of a scene, so English often uses is coming out of the shop or is leaving the shop.

Also, виходити is imperfective, so it can describe a process, repeated action, or a scene in progress.

А often links two parts of a sentence with a slight contrast or shift of attention. It is very common in Ukrainian.

Here:

• Вона виходить з магазину з важким пакетом = She is coming out of the shop with a heavy bag
• а в пакеті — молоко і хліб = and/as for what is in the bag—milk and bread

Using а feels natural because the sentence moves from the woman and the bag to the contents of the bag. It is not a strong contrast like but, but more a gentle switch in focus.

In many contexts, а can be translated as:

• and
• while
• but

depending on the situation.

The dash stands where English would often use is/are.

So:

• а в пакеті — молоко і хліб literally feels like
• and in the bag — milk and bread

More natural English:

• and in the bag there are milk and bread
• and the bag contains milk and bread

In Ukrainian, the verb є (is/are) is often omitted in the present tense, especially in simple descriptive sentences. The dash helps mark that relationship clearly.

Yes, absolutely.

• а в пакеті — молоко і хліб
• а в пакеті є молоко і хліб

Both are grammatical.

The version with the dash is a little more compact and stylistically neat. The version with є is also very natural and may feel slightly more explicit to a learner because it clearly includes there is/there are.

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

So:

• магазин can mean a shop or the shop
• пакет can mean a bag or the bag
• молоко і хліб can mean milk and bread

Context tells you whether something is definite or indefinite. In this sentence, English naturally uses the shop and the bag because they are understood from the situation.

In Ukrainian, пакет often means a bag, especially a shopping bag or plastic bag.

Depending on context, it can also mean:

• packet
• package
• bag

In this sentence, because someone is coming out of a shop carrying it, bag is the most natural translation.

They are in the nominative case here.

That is because they are the things being identified as the contents of the bag:

• в пакеті — молоко і хліб

This is like saying:

• what is in the bag? Milk and bread.

So they stay in their basic dictionary forms:

• молоко
• хліб

Ukrainian word order is fairly flexible, but the chosen order affects emphasis and naturalness.

This sentence is natural because it first presents the action:

• Вона виходить з магазину з важким пакетом

and then adds extra information:

• а в пакеті — молоко і хліб

You could rearrange parts, but the meaning emphasis would shift. For example:

• З магазину вона виходить з важким пакетом
  puts more emphasis on from the shop
• Молоко і хліб — у пакеті
  emphasizes the contents

So the original version is a very normal narrative order.