Iznad krova je letjela vrana, a vjeverica je ostala na stablu.

Because iznad normally requires the genitive case in Croatian.

• krov = roof
• krova = of the roof / roof (genitive form after certain prepositions)

So:

• iznad krova = above the roof

Many Croatian prepositions force a specific case, and iznad is one of the prepositions that takes the genitive.

Because na can be followed by different cases depending on the meaning.

Here it means location: the squirrel was on the tree, not moving onto it. For location, na takes the locative case.

• stablo = tree
• na stablu = on the tree

Compare:

• na stablu = on the tree, at the tree (location)
• na stablo = onto the tree (motion toward a surface), which would use the accusative

So ostala na stablu means she remained in that location.

Yes. This is the usual Croatian past tense, often called the perfect.

It is made with:

1. the present tense of biti (to be)
2. the l-participle of the main verb

So:

• je letjela = was flying / flew
• je ostala = stayed / remained

Here:

• je = auxiliary is/has-type helper form
• letjela = past participle of letjeti
• ostala = past participle of ostati

Croatian uses this structure where English often just uses a simple past form.

Because in Croatian, the past participle agrees with the gender and number of the subject.

The subjects are:

• vrana = crow
• vjeverica = squirrel

Both are grammatically feminine singular nouns, so the participles are also feminine singular:

• letjela
• ostala

If the subject were masculine, you would get forms like:

• je letio
• je ostao

This agreement happens even when talking about animals, because Croatian follows the noun’s grammatical gender.

This is because je is a clitic, and Croatian clitics usually go in the second position of the clause.

So in:

• Iznad krova je letjela vrana
• a vjeverica je ostala na stablu

the auxiliary je appears very early in the clause, not necessarily right before the participle in the English way.

This is very normal Croatian word order. You could also hear other word orders in some contexts, but the clitic still tends to stay in second position.

Yes. That would also be correct.

Croatian word order is more flexible than English because case endings show grammatical relationships. So both of these are possible:

• Iznad krova je letjela vrana
• Vrana je letjela iznad krova

The difference is mainly focus and style:

• Iznad krova je letjela vrana emphasizes the location first: above the roof
• Vrana je letjela iznad krova starts with the subject: the crow

So the original sentence is natural, but not the only possible order.

A often translates as and, but it has a slightly different feel from i.

In this sentence:

• ..., a vjeverica je ostala na stablu.

a links two clauses while also suggesting a contrast or change of scene:

• the crow was flying,
• while / and meanwhile the squirrel stayed on the tree.

So a is often like:

• and
• while
• whereas

depending on context.

If you used i, it would sound more like simple addition. A is a better choice here because it contrasts the two actions.

Yes, aspect is involved.

• letjeti is imperfective
• ostati is perfective

In this sentence:

• je letjela presents the crow’s flying as an ongoing action or scene in the past
• je ostala presents the squirrel’s staying/remained action as a completed result: she stayed/remained there

This combination is very natural:

• one background or continuing action: letjela
• one completed state-change/result: ostala

Croatian aspect is not exactly the same as English tense, but it strongly affects how the action is viewed.

They are not the same.

• ostala comes from ostati = to remain, stay, be left
• bila comes from biti = to be

So:

• vjeverica je ostala na stablu = the squirrel stayed/remained on the tree
• vjeverica je bila na stablu = the squirrel was on the tree

Ostala suggests that the squirrel continued to be there or did not leave. It has a stronger sense of remaining in place.

Because Croatian has no articles.

English says:

• the roof
• a crow
• the squirrel
• the tree

Croatian simply uses the noun without an article:

• krov
• vrana
• vjeverica
• stablo

Whether something is a crow or the crow is understood from context.

That is why the Croatian sentence can look more compact than the English one.

Vrana is a grammatically feminine noun, and vjeverica is too.

That means the grammar treats them as feminine:

• vrana je letjela
• vjeverica je ostala

This is about grammatical gender, not necessarily biological sex. Even if you are talking about a male animal, the noun itself may still be grammatically feminine.

This is common in Croatian and many other Slavic languages.

In this sentence, the comma is necessary and correctly used.

Croatian normally puts a comma before a when it connects two independent clauses:

• Iznad krova je letjela vrana, a vjeverica je ostala na stablu.

Each side has its own subject and verb:

• vrana je letjela
• vjeverica je ostala

So the comma helps separate the two clauses clearly.

These words can be tricky for English speakers because of the consonant-and-j combinations.

A rough guide:

• vjeverica ≈ vyeh-veh-ree-tsa
• letjela ≈ let-ye-la

A few helpful points:

• j in Croatian is pronounced like English y in yes
• lj, nj, dj/tj-type combinations can feel unusual at first
• every written vowel is pronounced clearly

So:

• vje- starts with something like vye-
• -jela in letjela sounds roughly like -yela

The exact pronunciation depends a bit on dialect and speech speed, but that approximation is good for a learner.