Летом мы покупаем черешню, а осенью — тыкву.

Because here they are being used as time expressions, and Russian normally puts season words in the instrumental case in this pattern:

• лето → летом
• осень → осенью

This instrumental form often means in summer, in autumn, etc.

You will see the same with other seasons:

• зима → зимой
• весна → весной

So летом and осенью are very natural adverb-like forms meaning during summer and during autumn.

Russian often does not use a preposition with seasons in this kind of general time expression. The instrumental case alone carries that meaning.

So Russian says:

• летом = in summer
• осенью = in autumn
• зимой = in winter
• весной = in spring

This is one of those places where Russian is more compact than English.

They are the direct objects of покупаем (we buy), so they go in the accusative case.

Both nouns are feminine singular, so their endings change like this:

• черешня → черешню
• тыква → тыкву

That is the normal accusative singular pattern for many feminine nouns ending in -а or -я.

Because Russian, like English, uses the present tense for habitual or repeated actions.

So мы покупаем here means something like:

• we buy
• we usually buy
• we buy as a regular habit

It is not necessarily talking about what is happening right this second. It describes a general pattern.

А is often used to show a contrast between two parallel ideas.

Here the structure is:

• Летом ... черешню
• а осенью ... тыкву

So а works well because it sets up a neat comparison: one thing in summer, another in autumn.

Very roughly:

• и = and
• но = but
• а = and/but, with contrast or comparison

In this sentence, а is the most natural choice.

The dash shows that some words are left out because they are already understood.

The full version would be:

Летом мы покупаем черешню, а осенью мы покупаем тыкву.

Russian often omits repeated words in the second part of a sentence if they are obvious. The dash helps mark that omission.

So after осенью, the missing idea is basically мы покупаем.

Yes, Russian could leave it out:

Летом покупаем черешню, а осенью — тыкву.

Russian often drops subject pronouns when the verb ending already makes the subject clear. Since покупаем clearly means we buy, мы is not strictly necessary.

Including мы can sound a bit clearer, more explicit, or slightly more contrastive, but both versions are possible.

No, Russian word order is fairly flexible. This sentence puts the season words first to highlight the contrast:

• Летом ...
• осенью ...

That makes the seasonal contrast feel especially clear.

Other word orders are possible, for example:

Мы летом покупаем черешню, а осенью — тыкву.

The original version is very natural because it foregrounds when each thing is bought.

This is a useful vocabulary point.

• черешня usually means sweet cherry
• вишня usually means sour/tart cherry

English often just says cherry, so learners may not notice the distinction at first. Russian commonly keeps them separate.

So the sentence specifically uses черешню, not just any cherry in a broad sense.

A natural stressed version is:

Ле́том мы покупа́ем чере́шню, а о́сенью — ты́кву.

A few notes:

• Ле́том: stress on the first syllable
• покупа́ем: stress on -па-
• чере́шню: stress on -ре́-
• о́сенью: stress on the first syllable
• ты́кву: stress on the first syllable

If you are reading aloud, the voice usually rises a little on the contrast and the dash creates a small pause before тыкву.