На сосне сидела белка и смотрела на нас сверху.

Because на here means on in the sense of location, and after на with a static location, Russian uses the prepositional case.

• dictionary form: сосна = pine tree
• prepositional singular: сосне

So:

• на сосне = on the pine tree

This is a very common pattern:

• на столе = on the table
• на крыше = on the roof
• на сосне = on the pine tree

Russian usually uses на with trees when something is sitting or located on them, especially animals and birds.

So Russian naturally says:

• белка сидела на сосне
• literally: the squirrel was sitting on the pine tree

An English speaker might expect in the tree, but Russian conceptualizes this more as on the tree. If you said в сосне, it would sound more like inside the pine tree, which is not the intended meaning here.

Because Russian past tense agrees with the subject in gender and number.

The subject is белка = squirrel, and белка is a feminine singular noun, so the verb must also be feminine singular:

• сидел = masculine singular
• сидела = feminine singular
• сидело = neuter singular
• сидели = plural

The same thing happens with смотрела.

Both verbs refer to the same subject, белка.

So Russian makes both past-tense verbs agree with that feminine subject:

• белка сидела
• белка смотрела

This is completely normal in coordinated verbs:

• Девочка сидела и читала. = The girl was sitting and reading.
• Белка сидела и смотрела. = The squirrel was sitting and looking.

Because the subject stays the same. In English, we often do the same thing:

• The squirrel was sitting and looking at us.

Russian likewise does not need to repeat белка:

• На сосне сидела белка и смотрела на нас сверху.

Repeating the subject would be possible only for emphasis or contrast, but it would sound unnecessary here.

Because the verb смотреть normally takes на + accusative when it means to look at.

So:

• смотреть на кого? на что?
• смотреть на нас = to look at us

The word нас is the accusative form of мы.

Compare:

• Я смотрю на тебя. = I’m looking at you.
• Она смотрела на нас. = She was looking at us.

You normally do not say смотреть нас.

Сверху means from above or from up above.

In this sentence it adds the idea that the squirrel was looking at us down from the tree, from a higher position.

So:

• смотрела на нас = was looking at us
• смотрела на нас сверху = was looking at us from above

Without сверху, the sentence would still be grammatical, but it would lose that extra spatial detail.

Because the sentence describes an ongoing scene, not single completed actions.

Imperfective verbs are commonly used for:

• background description
• ongoing action
• repeated or continuous action

Here the sentence paints a picture:

• the squirrel was sitting
• and was looking at us

That is why imperfective сидеть and смотреть are natural.

If you used a perfective verb like посмотрела, the meaning would shift more toward looked once / took a look rather than was looking.

Both are grammatical, but the emphasis is a little different.

• Белка сидела на сосне. = more neutral, straightforward
• На сосне сидела белка. = sets the scene first, then introduces the squirrel

Russian word order is more flexible than English, and speakers often move the location to the front for narrative effect or emphasis.

So this sentence feels a bit like:

• Up in the pine tree, a squirrel was sitting...

Yes. Russian does not rely on word order as strictly as English does. You identify the subject mainly by case and by the overall structure.

Here:

• белка is in the nominative case, so it is the subject
• сидела agrees with it in feminine singular
• нас is not the subject; it is the object of на

So even though белка comes after the verb, it is still the subject.

Russian has no articles like a or the.

So белка can mean:

• a squirrel
• the squirrel

The exact meaning depends on context.

In a sentence like this, English could translate it either way depending on the surrounding text:

• A squirrel was sitting in the pine tree...
• The squirrel was sitting in the pine tree...

Russian leaves that distinction to context rather than marking it with an article.

The main word stress is:

• На соснé сидéла бéлка и смотрéла на нас свéрху.

A learner should especially notice:

• соснé — stress on the last syllable
• сидéла — stress on де
• бéлка — stress on the first syllable
• смотрéла — stress on ре
• свéрху — stress on the first syllable

Correct stress matters a lot in Russian, because it is not always predictable.