У нашего старого кота длинные усы, сильные лапы, и он громко мурлычет, когда я сажусь рядом.

Russian often expresses possession with у + genitive, literally something like at our old cat = our old cat has.

So:

• У нашего старого кота длинные усы
  literally: At our old cat, there are long whiskers
• natural English: Our old cat has long whiskers

This structure is extremely common in Russian when talking about what someone has:

• У меня есть брат = I have a brother
• У собаки длинный хвост = The dog has a long tail

Russian can sometimes phrase things differently, but у + genitive is one of the most standard ways to say has/have.

Because the preposition у requires the genitive case.

Base forms:

• наш = our
• старый = old
• кот = cat

After у, they become genitive:

• нашего
• старого
• кота

So:

• у нашего старого кота = of/at our old cat

All three words match each other grammatically:

• masculine
• singular
• genitive

This is a very common pattern: a preposition forces a case, and the adjective/pronoun/noun all change accordingly.

Because у takes the genitive, not the dative.

For masculine singular nouns like кот, the genitive form is often -а:

• кот → кота

But коту is dative:

• дать коту еду = to give food to the cat

So in this sentence:

• у кота = correct
• у коту = incorrect

Because they describe plural nouns:

• усы = whiskers / moustache
• лапы = paws

In Russian, adjectives agree with the nouns they describe in:

• gender
• number
• case

So:

• длинные усы = long whiskers
• сильные лапы = strong paws

If the noun were singular, the adjective would also be singular:

• длинный хвост = long tail
• сильная лапа = strong paw
• длинное ухо = long ear

Russian normally talks about a cat’s whiskers as усы, a plural noun.

• ус can mean a single whisker or a moustache hair
• усы usually means whiskers or moustache as a set

So for an animal, длинные усы is the normal phrase.

English also usually says whiskers in the plural, so this matches pretty well.

Russian often leaves out есть in sentences describing what someone has, especially in the present tense.

So:

• У кота длинные усы, сильные лапы
  = The cat has long whiskers and strong paws

You could sometimes use есть, but here it is not necessary. In fact, without есть, the sentence sounds very natural and descriptive.

Also, Russian often lists possessed features directly after у + genitive:

• У неё голубые глаза = She has blue eyes
• У дома большая крыша = The house has a big roof

Russian often includes the subject pronoun when starting a new clause or when shifting to a new action.

The sentence has two descriptive parts:

• У нашего старого кота длинные усы, сильные лапы
• и он громко мурлычет...

Adding он makes the second clause clear and natural:

• first: description of his features
• second: what he does

Russian sometimes omits pronouns, but here он helps structure the sentence and sounds normal.

Мурлычет is the 3rd person singular present tense of мурлыкать / мурлыкать-like verb form мурлыкать / мурлыкать? Actually the standard verb is мурлыкать or мурчать, and мурлычет is the present form meaning purrs / is purring.

Here it agrees with он:

• я мурлычу = I purr
• ты мурлычешь = you purr
• он мурлычет = he purrs

Russian present tense often covers both:

• he purrs
• he is purring

The exact English translation depends on context.

Громко is an adverb meaning loudly.

It modifies the verb:

• мурлычет = purrs
• громко мурлычет = purrs loudly

This is straightforward adverb use. Compare:

• тихо говорит = speaks quietly
• быстро бежит = runs quickly
• громко мурлычет = purrs loudly

Because когда я сажусь рядом is a subordinate clause: when I sit down nearby / when I sit next to him.

Russian normally separates subordinate clauses with a comma:

• Он улыбается, когда меня видит = He smiles when he sees me
• Я читаю, пока он спит = I read while he sleeps

So here:

• ..., когда я сажусь рядом

That comma is required in standard Russian punctuation.

Because сажусь means I sit down / I am taking a seat, while сижу means I am sitting.

This is an important distinction:

• сидеть = to be sitting
• садиться / сесть = to sit down, to take a seat

So the sentence means the cat purrs when the speaker sits down beside him, not simply while the speaker is already seated.

Compare:

• Когда я сажусь рядом, он мурлычет = When I sit down next to him, he purrs
• Когда я сижу рядом, он мурлычет = When I am sitting next to him, he purrs

Both are possible, but they mean slightly different things.

Because the verb is садиться, a reflexive verb.

In Russian, many verbs of movement or change of position can be reflexive:

• садить = to seat someone, to plant something
• садиться = to sit down oneself

So:

• я сажусь = I sit down
• он садится = he sits down

The -ся / -сь ending often shows that the action refers back to the subject.

Here сажусь is the 1st person singular form:

• я сажусь = I sit down

Because сажусь is imperfective/present, and it works naturally in a repeated or habitual situation after когда.

This sentence suggests something like:

• Whenever I sit down nearby, he purrs loudly

That repeated, general meaning often uses the imperfective present:

• когда я прихожу, он радуется = when I come, he gets happy
• когда я открываю окно, кошка прыгает = when I open the window, the cat jumps

Сяду is future perfective:

• когда я сяду рядом = when I sit down / once I have sat down

That would point more to a single completed future event. It is grammatical in the right context, but it would change the nuance.

Рядом means nearby, close by, or next to.

In this sentence:

• когда я сажусь рядом
  = when I sit down nearby / when I sit down next to him

Because the cat is the obvious reference point, Russian does not need to repeat it.

You could make it more explicit:

• когда я сажусь рядом с ним = when I sit down next to him

But leaving с ним out is natural if the meaning is already clear from context.

It can be, but Russian often omits information that is obvious from context.

Since the whole sentence is about the cat, рядом alone is enough for many speakers:

• когда я сажусь рядом = when I sit down nearby / next to him

If you want to be fully explicit, you can say:

• когда я сажусь рядом с ним
• когда я сажусь рядом с котом

So the shorter version is natural because the listener already knows who nearby refers to.

Yes, Russian word order is flexible, but the given version sounds natural and neutral.

Current order:

• У нашего старого кота длинные усы, сильные лапы, и он громко мурлычет, когда я сажусь рядом.

This flows as:

1. possession/description
2. more description
3. action
4. time clause

You could rearrange parts for emphasis, for example:

• Наш старый кот громко мурлычет, когда я сажусь рядом. У него длинные усы и сильные лапы.
• Когда я сажусь рядом, наш старый кот громко мурлычет.

But the original is a very normal descriptive sentence.

Yes, here it is normal because и joins two clauses, and the second clause has its own subject and verb:

• first clause: У нашего старого кота длинные усы, сильные лапы
• second clause: он громко мурлычет

So the comma helps separate the two parts.

Inside the first clause, длинные усы, сильные лапы is also a list, so the commas are natural there too.

Yes. Once the cat has been introduced, Russian could say:

• У нашего старого кота длинные усы и сильные лапы. Он громко мурлычет, когда я сажусь рядом.
• or later: У него длинные усы и сильные лапы.

But in the original sentence, using у нашего старого кота right away is just the full descriptive way to introduce him.