Летом мы часто замораживаем ягоды, а зимой постепенно размораживаем их для компота.

With seasons, Russian very often uses special time expressions like летом, зимой, весной, осенью with no preposition.

So:

• летом = in summer
• зимой = in winter

These forms come historically from the instrumental case, but for a learner it is often easiest to remember them as fixed adverb-like time words.

You would normally say:

• Летом жарко. = It’s hot in summer.
• Зимой холодно. = It’s cold in winter.

Using в here would usually sound wrong in standard Russian.

Russian often uses the present tense of an imperfective verb to describe a habitual/repeated action.

So мы часто замораживаем ягоды means:

• we often freeze berries
• or more naturally, in summer we often freeze berries

It does not mean only right now, at this exact moment. It describes a repeated practice or routine.

The same applies to размораживаем:

• зимой постепенно размораживаем = in winter we gradually thaw them

This is a typical use of the imperfective present for habits, regular actions, and general practices.

They are imperfective because the sentence describes a repeated process, not one single completed event.

Compare:

• замораживаем / размораживаем = imperfective
  used for habits, repeated actions, processes
• заморозим / разморозим = perfective
  used for a single completed action, often future in meaning

In this sentence, the idea is:

• in summer, we do this often
• in winter, we do this gradually, again and again

That is why imperfective is the natural choice.

If you said заморозим or разморозим, it would sound more like one specific completed action, not a seasonal routine.

Because ягоды is the accusative plural of an inanimate noun, and for inanimate plural nouns, the accusative looks the same as the nominative.

So:

• nominative plural: ягоды
• accusative plural: ягоды

That is why:

• замораживаем ягоды = we freeze berries

You might see ягод in other contexts, for example:

• много ягод = many berries
• нет ягод = there are no berries

But after a normal transitive verb like замораживать, the direct object here is accusative plural: ягоды.

А often links two parts of a sentence with a sense of contrast, switching, or comparison, while и is a more neutral and.

Here the sentence contrasts two seasonal actions:

• Летом we freeze berries,
• а зимой we thaw them.

So а works very naturally because the speaker is shifting from summer to winter.

Using и would be grammatically possible in some contexts, but а sounds better because it highlights the contrast between the two times and the two related actions.

Их means them, referring back to ягоды.

So:

• размораживаем их = we thaw them

In Russian, object pronouns are often included when you want to clearly refer back to something already mentioned. Here it helps avoid repeating ягоды.

You could also say:

• Летом мы часто замораживаем ягоды, а зимой постепенно размораживаем для компота.

This may be understood, but их makes the sentence clearer and more natural because it explicitly tells you what is being thawed.

Because the preposition для always takes the genitive case.

So:

• для = for
• компот = nominative
• для компота = for compote

This phrase means something like:

• for making compote
• to use in compote

A few more examples:

• для чая = for tea
• для супа = for soup
• для детей = for children

So the genitive here is required by the preposition для.

Постепенно means gradually.

In this sentence, it suggests that during winter the berries are not all thawed at once. Instead, they are thawed little by little, probably in portions, whenever needed for compote.

So the sense is:

• in summer, we freeze berries
• in winter, we thaw them gradually, as needed

It makes the situation more realistic and natural.

Yes. Russian word order is flexible, and changes often affect emphasis rather than basic meaning.

The original sentence is natural:

• Летом мы часто замораживаем ягоды, а зимой постепенно размораживаем их для компота.

You could also hear:

• Мы летом часто замораживаем ягоды, а зимой постепенно размораживаем их для компота.
• Ягоды мы летом часто замораживаем, а зимой постепенно размораживаем их для компота.

These alternatives are possible, but they emphasize different parts of the sentence.

For learners, the original version is a very good neutral model:

• time expression first,
• subject next,
• verb,
• object.

Yes. The prefixes help show opposite directions of the action:

• замораживать = to freeze
• размораживать = to defrost / thaw

Here:

• за- in замораживать helps form the verb meaning to freeze
• раз- in размораживать suggests undoing that state, so to unfreeze / thaw

These two verbs form a very natural pair:

• заморозить / замораживать
• разморозить / размораживать

This kind of prefix-based pairing is common in Russian vocabulary and can be very useful to notice.

Then Russian would normally use perfective future forms.

For example:

• Летом мы заморозим ягоды, а зимой разморозим их для компота.

This means something more like:

• In the summer we will freeze the berries, and in the winter we will thaw them for compote.

That sounds like a particular plan or one completed set of actions.

By contrast, the original sentence with замораживаем / размораживаем describes a regular yearly habit.