Когда мы уезжали, белка всё ещё сидела на сосне, а петух снова кричал у забора.

Because уезжали is imperfective past, so it presents the action as a process: when we were leaving / as we were leaving.

That fits this sentence well, because the squirrel and the rooster are being described as doing something during that time.

Compare:

• Когда мы уезжали... = When we were leaving...
• Когда мы уехали... = When we had left / after we left...

So уезжали gives background timing, while уехали would sound more like the departure was completed first.

For the same general reason: they describe actions or states as ongoing at that moment in the past.

• белка сидела = the squirrel was sitting
• петух кричал = the rooster was crowing / shouting

The sentence is painting a scene in progress.

If you used perfective verbs instead, the meaning would change:

• села = sat down
• крикнул = gave a single shout

Those would sound like single completed events, not background actions.

It means something like:

• When we were leaving
• As we were leaving

So it sets the time frame for the rest of the sentence.

The structure is:

• Когда мы уезжали = time clause
• белка всё ещё сидела на сосне, а петух снова кричал у забора = main description

In other words: during the time of our departure, these two things were happening.

Всё ещё means still.

So:

• белка всё ещё сидела на сосне = the squirrel was still sitting in/on the pine tree

It suggests that the squirrel had already been there before, and it had not moved yet.

This is a very common Russian phrase:

• Я всё ещё жду. = I am still waiting.
• Он всё ещё дома. = He is still at home.

Also, in normal typing you will often see еще instead of ещё. Both mean the same thing; the dots over ё are often omitted in everyday writing.

Yes, опять could be used here too.

• снова = again
• опять = again

In many contexts they are interchangeable.

So:

• петух снова кричал
  and
• петух опять кричал

can both mean the rooster was crowing again.

A rough tendency:

• снова can sound a bit more neutral or literary
• опять can sometimes sound more conversational, and in some contexts it can carry a feeling like again, annoyingly

But this is only a tendency, not a hard rule.

А often connects two clauses while also showing a contrast of topic or a shift in attention.

Here it works like:

• the squirrel was still sitting on the pine, and meanwhile / whereas the rooster was crowing again by the fence

It is not a strong contradiction, so но would be too strong.
And и would simply add another fact without that slight contrast or switch.

Very roughly:

• и = and
• а = and / while / whereas
• но = but

So а is a very natural choice here.

It is prepositional case after на when talking about location.

Base form:

• сосна = pine tree

After на for location:

• на сосне = on the pine tree / in the pine tree

This is the pattern:

• на + prepositional for location

Examples:

• на столе = on the table
• на улице = on the street / outside
• на дереве = in/on the tree
• на сосне = in/on the pine tree

With birds and small animals, Russian often uses на дереве / на сосне where English might prefer in the tree.

After у, Russian uses the genitive case.

Base form:

• забор = fence

Genitive singular:

• забора

So:

• у забора = by the fence / near the fence

This is a very common pattern:

• у дома = by the house
• у окна = by the window
• у дороги = by the road

In this sentence, у забора tells you where the rooster was.

Yes. Возле забора would also mean near the fence.

Both are possible:

• у забора
• возле забора

A rough difference:

• у is extremely common and compact
• возле is a bit more explicitly near/beside

In this sentence, у забора sounds very natural.

Because in Russian, past-tense verbs agree with gender and number.

• белка is feminine, so: сидела
• петух is masculine, so: кричал
• мы is plural, so: уезжали

This is an important feature of Russian past tense.

Examples:

• мальчик читал = the boy was reading
• девочка читала = the girl was reading
• дети читали = the children were reading

Yes, Russian word order is fairly flexible.

The sentence as written sounds natural and neutral:

• белка всё ещё сидела на сосне
• петух снова кричал у забора

But some parts could be moved for emphasis:

• Белка сидела всё ещё на сосне
• Снова петух кричал у забора
• У забора петух снова кричал

These are all possible in the right context, but they shift emphasis or sound less neutral.

So the original order is a good standard way to say it.

There are two reasons.

1. After the subordinate clause:
   • Когда мы уезжали, ...

Russian puts a comma after a time clause like Когда мы уезжали.

2. Before а:
   • ..., а петух снова кричал у забора

The conjunction а joins two clauses, and Russian normally puts a comma before it.

So the punctuation is completely standard.

Yes, кричал works.

Literally, кричать means to shout / cry out, but with animals it can be used more broadly for the sounds they make, depending on context.

For a rooster, Russian can also use:

• кукарекал = was crowing

That is more specific.
So:

• петух снова кричал = the rooster was crying out / crowing again
• петух снова кукарекал = the rooster was crowing again

The sentence uses the more general verb кричал, which is still natural.

Because Russian has no articles.

So:

• белка can mean a squirrel or the squirrel
• петух can mean a rooster or the rooster
• забор can mean a fence or the fence

The context tells you which meaning is intended.

That is why the English translation has to choose a or the, but the Russian sentence does not.

Historically related, yes, but in the phrase всё ещё you should learn it as a set expression meaning still.

By itself:

• всё can mean everything or all

But in:

• всё ещё

the whole phrase means:

• still

So it is better not to translate it word by word in this sentence.