На аллее было тихо, но над лужайкой всё равно летала одна ворона.

Russian uses на with some open spaces and surfaces, and аллея is understood as a path or walkway in an open area, so на аллее is the normal way to say on the avenue / on the path / along the tree-lined walk.

Also, after на for location, Russian uses the prepositional case, so:

• аллея → на аллее

If you were talking about motion onto that place, you would use the accusative:

• на аллею = onto the avenue/path

This is an impersonal construction, very common in Russian.

Russian often says:

• было тихо
• literally something like was quiet

In natural English, we add a dummy subject and say it was quiet, but Russian does not need that kind of it.

Because there is no real subject here, the past tense verb appears in the default neuter singular form:

• было

So было is not agreeing with аллее.
На аллее is just a location phrase, not the subject.

Because тихо here is not a regular adjective describing a noun directly. It is a predicative word meaning quiet as a general state or atmosphere.

So:

• На аллее было тихо = It was quiet on the avenue/path

If you wanted to describe the alley itself as a noun with an adjective, you could say something like:

• Аллея была тихой

That is grammatical, but it feels a bit different.
Было тихо is the more natural way to describe the overall scene or ambience.

The preposition над means above / over, and it normally takes the instrumental case.

So:

• лужайка → над лужайкой

That is why you see the ending -ой.

So:

• над лужайкой = over the lawn / above the grassy patch

This is a very common pattern:

• над домом = over the house
• над рекой = over the river
• над головой = over the head

Here всё равно means something like:

• all the same
• anyway
• still
• nevertheless

So the idea is:

• it was quiet,
• but despite that, one crow was still flying over the lawn

Important: всё равно can also mean it doesn’t matter, depending on context. For example:

• Мне всё равно = I don’t care / It makes no difference to me

But in this sentence, it means despite what was just said.

This is a very important Russian verb distinction.

• лететь = to fly in one direction, as a single movement
• летать = to fly around, fly repeatedly, fly back and forth, or fly in a general area

In the sentence, the crow is not presented as flying from point A to point B. It is just moving around over the lawn, so летала is natural.

Compare:

• Ворона летела к дереву = The crow was flying toward the tree
  → one directional flight

• Ворона летала над лужайкой = The crow was flying around over the lawn
  → movement within an area, not one straight route

So летала fits the image much better.

In the past tense, Russian verbs agree with the subject in gender and number.

The subject here is:

• ворона

And ворона is a feminine noun, so the past tense form is:

• летала

Compare:

• ворона летала
• ворон летал
• птицы летали

This is grammatical gender, not necessarily biological sex.
Even if the actual bird’s sex is unknown, the noun ворона is grammatically feminine, so the verb is feminine too.

Because the numeral one must agree in gender with the noun.

• masculine: один
• feminine: одна
• neuter: одно

Since ворона is feminine, you need:

• одна ворона

Examples:

• один дом
• одна книга
• одно окно

Literally, одна ворона means one crow.

Because Russian has no articles, sometimes English a/an is translated with no extra word at all. So Russian could simply say:

• над лужайкой летала ворона

That would mean a crow was flying over the lawn.

By adding одна, the sentence gives extra emphasis to the number:

• just one crow
• a single crow

That makes sense here, because it contrasts with the quietness of the scene.

Russian word order is flexible, and word order often shows focus and information structure, not just grammar.

Putting одна ворона at the end makes it the most noticeable new piece of information. The sentence first sets the scene:

• На аллее было тихо
• но над лужайкой всё равно летала...

And then it reveals what was there:

• одна ворона

That ending gives a natural, slightly dramatic or observational feel: everything was quiet, but there was still that one crow.

Other word orders are possible, but they would shift the emphasis.

Because всё равно modifies the whole action and expresses the speaker’s attitude toward it: despite everything, still, anyway.

So:

• но над лужайкой всё равно летала одна ворона

means roughly:

• but over the lawn, one crow was still flying anyway

Placing всё равно before the verb is a very normal position in Russian. It helps show that the whole event is happening in spite of the quietness mentioned earlier.

Because но means but and joins two clauses:

• На аллее было тихо
• но над лужайкой всё равно летала одна ворона

In Russian, a comma is normally used before но when it connects two separate clauses.

So the punctuation here is completely standard.