Кошка залезла на дерево.

A natural pronunciation is:

Ко́шка зале́зла на де́рево.

Stress:

• ко́шка — stress on the first syllable
• зале́зла — stress on ле
• де́рево — stress on the first syllable

Rough English-style approximation:

• KOSH-ka za-LEZ-la na DYE-re-va

A few pronunciation notes:

• ш sounds like sh in ship
• ж would sound like s in measure, but there is no ж here
• unstressed о is often reduced in speech, so кошка does not sound like a strong ko

Because it is past tense feminine singular.

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

• masculine: залез
• feminine: залезла
• neuter: залезло
• plural: залезли

The subject here is кошка, which is a feminine noun, so the verb is залезла.

Compare:

• Кот залез на дерево. — A male cat climbed up the tree.
• Кошка залезла на дерево. — A female cat climbed up the tree.

Залезла is the past tense of залезть, which usually means something like:

• climbed up
• got up onto
• scrambled onto

With на дерево, it means the cat climbed up onto a tree.

This verb often suggests climbing with effort, using the body, paws, hands, etc. It feels more physical than a neutral verb like went.

So this sentence is more specifically The cat climbed up a/the tree rather than just The cat went to the tree.

This is an aspect question.

• лезть = imperfective
• залезть = perfective

In this sentence, залезла is perfective, so it presents the action as completed: the cat successfully got up the tree.

Very roughly:

• Кошка лезла на дерево. — The cat was climbing up the tree / the cat climbed up the tree (focusing on the process)
• Кошка залезла на дерево. — The cat climbed up the tree (focusing on the result)

English often does not mark this distinction as clearly as Russian does, so learners often need to pay extra attention to aspect.

На often means onto / on / onto the surface of.

With verbs of motion, на + accusative usually expresses movement onto something:

• на стол — onto the table
• на крышу — onto the roof
• на дерево — onto/up a tree

So Кошка залезла на дерево literally has the idea of the cat moving onto/up the tree.

Russian often uses на with things that are understood as surfaces or places one can get onto.

Because this sentence describes motion toward a destination, not location.

Russian often contrasts:

• на + accusative = motion onto something
• на + prepositional = location on something

So:

• Кошка залезла на дерево. — The cat climbed onto/up the tree.
• Кошка сидит на дереве. — The cat is sitting in/on the tree.

This is a very common Russian pattern:

• на стол — onto the table
• на столе — on the table
• на дерево — onto the tree
• на дереве — in/on the tree

With trees, English usually says in the tree, but Russian commonly says на дереве.

It is accusative here, but for this noun the accusative singular looks exactly like the nominative singular.

Дерево is:

• neuter
• inanimate
• singular

For most inanimate nouns in Russian, the accusative form is the same as the nominative.

So:

• nominative: дерево
• accusative: дерево

Even though the form looks unchanged, its function in the sentence is still accusative because it follows на in a motion meaning.

Yes — literally, Russian has the idea of movement onto/up the tree.

But in natural English, the best translation is often:

• The cat climbed up the tree.

Russian and English describe trees differently:

• Russian commonly says на дереве / на дерево
• English commonly says in the tree / up the tree

So you should not translate word-for-word too rigidly here. The Russian structure is natural even if the most natural English wording is different.

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

The neutral order here is:

• Кошка залезла на дерево.

But you could also hear:

• На дерево залезла кошка.
• Залезла кошка на дерево.

The basic meaning stays the same, but the emphasis changes:

• Кошка залезла на дерево. — neutral statement
• На дерево залезла кошка. — emphasizes onto the tree
• Залезла кошка на дерево. — can sound more narrative or stylistically marked

English usually has much less freedom here.

Russian has no articles.

So Кошка залезла на дерево could mean:

• The cat climbed up the tree
• A cat climbed up a tree
• The cat climbed up a tree

The exact meaning depends on context.

Russian speakers use context, word order, and sometimes extra words to show whether something is definite or indefinite.

You would normally say:

Кошка на дереве.
or
Кошка сидит на дереве.

This shows the important difference between:

• movement: Кошка залезла на дерево.
• location: Кошка на дереве. / Кошка сидит на дереве.

So:

• на дерево = onto/up the tree
• на дереве = in/on the tree

This contrast is one of the key grammar points in the sentence.