Птица перелетела через реку и села на дерево.

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

The subject is птица (bird), which is a singular feminine noun, so the past-tense verb takes the feminine form:

• masculine: перелетел
• feminine: перелетела
• neuter: перелетело
• plural: перелетели

The same thing happens with села: it is also feminine singular because the subject is still птица.

Because the preposition через requires the accusative case.

The dictionary form is река (river), but in the accusative singular it becomes реку.

So:

• nominative: река
• accusative: реку

This is very common in Russian: prepositions often force a specific case.

The prefix пере- often gives the idea of crossing, going across, or moving from one side to another.

So:

• лететь = to fly
• перелететь = to fly across / fly over / cross by flying

In this sentence, перелетела через реку means the bird flew from one side of the river to the other.

Because перелетела is perfective, while летела is imperfective.

Here the sentence describes a completed action: the bird successfully crossed the river and then landed. That is why perfective fits well.

Compare:

• Птица летела через реку. = The bird was flying over/across the river.
  This focuses more on the process.
• Птица перелетела через реку. = The bird flew across the river.
  This focuses on the completed result.

Russian uses aspect very often to show whether an action is seen as ongoing/process-like or completed/result-like.

Because сесть and сидеть mean different things.

• сесть / села = to sit down, to take a seat, to land, to perch
• сидеть / сидела = to be sitting

So in this sentence, the bird is changing position: first it flew, then it landed/perched on the tree. That is why села is used.

If you said сидела, it would mean the bird was sitting on the tree, not that it landed there.

Because Russian distinguishes between motion toward a place and location at a place.

With на:

• на + accusative = motion onto/to
• на + prepositional = location on/at

So:

• села на дерево = landed onto the tree
• сидела на дереве = was sitting on the tree

Here the bird moves onto the tree, so Russian uses на дерево.

It is accusative — it just happens to look the same as the nominative.

For many inanimate neuter nouns, the nominative and accusative singular forms are identical.

So:

• nominative: дерево
• accusative: дерево

That is normal Russian grammar, not an exception just for this sentence.

Yes, it is natural.

In Russian, сесть на дерево can be used for a bird and means that it landed/perched on the tree. It does not usually mean the bird is somehow sitting on the top of the whole tree in a literal English sense. Russian often expresses this more simply than English.

If someone wanted to be more specific, they could say:

• села на ветку = landed on a branch

But села на дерево is perfectly understandable and normal.

Because the subject is already clear.

Russian often omits personal pronouns when they are unnecessary. After Птица перелетела через реку, it is obvious that the same subject continues, so села alone is enough.

Adding она is possible, but it would usually sound more emphatic or less natural in a simple sentence like this.

Yes. It connects the two actions:

1. the bird flew across the river
2. the bird landed on the tree

It also suggests a natural sequence: first one completed action, then the next.

Yes. Russian word order is more flexible than English word order.

The sentence as given is a very neutral, natural order:

• Птица перелетела через реку и села на дерево.

You could change the order for emphasis, for example:

• Через реку птица перелетела и села на дерево.
• Птица через реку перелетела и села на дерево.

These are possible, but the original version is the most straightforward and neutral for a learner.

A natural stress pattern is:

пти́ца перелете́ла че́рез реку́ и се́ла на де́рево

Word by word:

• пти́ца
• перелете́ла
• че́рез
• реку́
• се́ла
• де́рево

Learning stress is important in Russian because it is not always predictable from spelling.