Я не люблю огірок узимку, але влітку він дуже смачний.

In this sentence, люблю is best understood as like.

Ukrainian любити can mean either to love or to like, depending on context. With food, drinks, activities, and habits, it very often means to like / to enjoy.

So:

• Я люблю каву. = I like coffee.
• Я люблю тебе. = I love you.

Here, because the object is огірок (cucumber), the natural translation is I don’t like cucumber in winter.

Here it most likely means cucumber in general, not just one specific cucumber.

Ukrainian often uses the singular to talk about a food item as a general category, especially in simple statements about taste or preference.

So Я не люблю огірок узимку is like saying:

• I don’t like cucumber in winter
• not necessarily I don’t like one cucumber

You could also say огірки (cucumbers) if you want. That would sound more concrete or more obviously plural:

• Я не люблю огірки взимку, але влітку вони дуже смачні.

Both are possible; the singular here is a normal generic use.

It is in the correct case already: the accusative.

The verb любити takes a direct object, so огірок is in the accusative case. But for many inanimate masculine singular nouns, the accusative form is the same as the nominative.

So:

• nominative: огірок
• accusative: огірок

That is why you do not see a visible change.

Compare that with an animate masculine noun:

• Я люблю брата. = I love my brother.

Here the accusative is different: брата, not брат.

So the short answer is: the case does change grammatically, but the form happens to look the same.

Because in modern Ukrainian these are commonly treated as fixed adverbial forms meaning in winter and in summer.

So:

• узимку / взимку = in winter
• влітку / улітку = in summer

They answer the question when? and function like adverbs in the sentence.

For a learner, it is easiest to remember them as whole expressions:

• узимку
• влітку
• навесні
• восени

Yes. Those variants are normal.

Ukrainian alternates у / в for smoother pronunciation. This is a very common sound-based pattern.

So all of these are standard:

• узимку / взимку
• влітку / улітку

Writers and speakers often choose the version that sounds better next to the surrounding words.

In your sentence:

• огірок узимку flows smoothly
• але влітку also flows smoothly

So the choice is mainly about euphony (pleasant sound), not a change in meaning.

Because in Ukrainian, the verb to be is usually omitted in the present tense.

So:

• він дуже смачний literally looks like he very tasty
• but it means it is very tasty

This is completely normal Ukrainian grammar.

Compare:

• Він смачний. = It is tasty.
• Вони смачні. = They are tasty.

English needs is/are, but Ukrainian usually does not in present-tense sentences like this.

Because він refers back to огірок, and огірок is a masculine singular noun.

In Ukrainian, pronouns agree with the grammatical gender and number of the noun they replace.

So:

• огірок is masculine
• therefore the pronoun is він = he/it

Even though English uses it for things, Ukrainian uses the ordinary gendered pronoun that matches the noun.

Examples:

• огірок → він
• книга → вона
• море → воно

Because смачний is an adjective agreeing with він / огірок.

Here the structure is:

• він = subject
• смачний = adjective describing that subject

Since огірок is masculine singular, the adjective is also masculine singular:

• він дуже смачний

If the noun were plural, the adjective would change:

• огірки дуже смачні

Смачно is a different form. It is usually an adverb or predicative word, as in:

• Тут смачно готують. = They cook tasty food here.
• Було смачно. = It was tasty.

So with він, смачний is the right choice.

Yes, you could, but the nuance is a little different.

• але = but, often a stronger contrast
• а = and / but / whereas, often a softer contrast or comparison

So:

• Я не люблю огірок узимку, але влітку він дуже смачний. = a clear contrast: I don’t like cucumber in winter, but in summer it’s very tasty.

A version with а would also be natural:

• Я не люблю огірок узимку, а влітку він дуже смачний.

That can feel a bit more like whereas in summer it is very tasty.

Both work; але just emphasizes the contrast more strongly.

No. Ukrainian word order is fairly flexible.

The sentence you have is natural and neutral:

• Я не люблю огірок узимку, але влітку він дуже смачний.

But you can move parts around to change emphasis.

For example:

• Узимку я не люблю огірок... puts more focus on in winter
• Але він улітку дуже смачний puts more focus on it

Even though the order can change, the original version sounds very normal and easy to understand.

Yes, you could say:

• Я не люблю огірок узимку, але влітку огірок дуже смачний.

That is grammatical, but it sounds more repetitive.

Using він is more natural because Ukrainian, like English, usually avoids repeating the same noun when the reference is already clear.

So:

• ...але влітку він дуже смачний
  sounds smoother and more natural than repeating огірок again.