По вечерам мы выпускаем кошку на балкон ненадолго, но не выпускаем её на улицу.

По вечерам means in the evenings or every evening / on evenings in general.

This is a common Russian pattern:

• по + dative plural can mean on / during / at different times of a repeated period
• so по вечерам = in the evenings
• compare:
  • по утрам = in the mornings
  • по ночам = at night, at nights
  • по выходным = on weekends

The noun вечер becomes вечерам, which is the dative plural form.

In Russian, the present tense of an imperfective verb is often used for habitual or repeated actions.

So мы выпускаем here does not mean only we are letting out right now. It means something like:

• we let the cat out
• we usually let the cat out
• we do this regularly

Because the sentence describes a habit, the present tense is exactly what Russian normally uses.

Выпускаем comes from выпускать, which is imperfective.

Imperfective is used here because the sentence is about:

• a repeated action
• a routine
• a general practice

If you wanted to talk about one completed action, Russian would more likely use the perfective verb выпустить.

For example:

• Сегодня вечером мы выпустим кошку на балкон. = This evening we will let the cat out onto the balcony.

But in your sentence, the meaning is habitual, so выпускаем is the natural choice.

Because кошка is the direct object of выпускаем, it has to be in the accusative case.

• nominative: кошка
• accusative: кошку

Since кошка is feminine singular, its accusative form is кошку.

This is exactly what Russian does with direct objects after transitive verbs like выпускать.

In the first clause, the noun is named directly:

• мы выпускаем кошку...

In the second clause, Russian naturally switches to the pronoun:

• но не выпускаем её...

That is very normal. English does the same thing:

• we let the cat out..., but we don’t let her/it out...

Using её avoids repeating кошку unnecessarily.

Also, её here is the accusative form of она.

Russian nouns have grammatical gender, and кошка is feminine. So the pronoun referring back to it is feminine too:

• кошка → она / её

In English, animals can be referred to as it, but people often say she/her for a known female animal or a pet. Russian is less flexible here because the grammar follows the noun’s gender.

Also, кошка usually means a female cat, although it can sometimes be used more generally in everyday speech for cat.

Because на can be used with different cases depending on meaning.

Here it expresses motion toward a place / onto a place, so it takes the accusative:

• на балкон = onto/to the balcony
• на улицу = out onto/to the street/outside

Compare that with location, where Russian would use the prepositional case:

• на балконе = on the balcony
• на улице = outside / on the street

So:

• выпускаем на балкон = let out onto the balcony
• кошка на балконе = the cat is on the balcony

Literally, на улицу means onto the street, but very often in Russian it means outside in a broader sense.

So in this sentence, it does not necessarily mean the cat is being taken to the road itself. It usually means:

• outside the apartment/building area
• outdoors
• out where it can roam freely

That is a very common Russian usage.

Ненадолго means for a short time, briefly, or not for long.

It is written as one word because it functions as a lexical adverb meaning briefly / for a little while, not just a fully separate logical negation.

So here:

• выпускаем кошку на балкон ненадолго = we let the cat out onto the balcony for a short while

If you separated it in another context, не надолго, that would usually create a stronger contrast or emphasis, something like not for long in a more literal sense. But the normal neutral spelling here is ненадолго.

Yes, Russian sometimes could use а in contrasting sentences, but но is more natural here because the second clause feels like a stronger but:

• we do let the cat out onto the balcony,
• but we do not let her outside.

So но emphasizes a clear opposition or restriction.

Very roughly:

• но = but
• а = and/but, often for contrast or comparison

In this sentence, но sounds straightforward and natural.

Yes, you could say:

• Мы по вечерам выпускаем кошку...

That is perfectly grammatical.

Russian word order is more flexible than English word order. The version with По вечерам first puts the time frame upfront, as the topic:

• As for evenings / In the evenings, we let the cat out...

That sounds very natural because the sentence is about a routine.

So both are possible, but the original order is especially good if you want to foreground when this happens.

Russian has no articles like English a and the.

So кошка / кошку can mean:

• a cat
• the cat

The context tells you which one is meant. In this sentence, it is clearly a specific cat that belongs to or is known to us, so English would naturally use the cat.

Russian leaves that to context instead of marking it with an article.

Её is pronounced roughly like yi-YO.

Important points:

• ё always represents a yo sound
• the stress here is on the second syllable: её

In real Russian writing, native texts often omit the dots and write ее instead of её, even though the pronunciation is still её. Learners often find this confusing.

So:

• written carefully: её
• often seen in normal text: ее
• pronounced: ye-YO / yi-YO

Usually:

• кот = male cat
• кошка = female cat

But in everyday speech, кошка can also be used more loosely when people just mean cat, especially if the animal is a pet and its sex is known or not very important to the speaker.

In your sentence, using кошка strongly suggests the cat is female, which also matches the pronoun её.