У входа небольшое крыльцо, и на этом крыльце часто сидит старая кошка.

У входа means by the entrance / at the entrance.

After the preposition у, Russian normally uses the genitive case, so:

• вход = entrance
• у входа = by the entrance

This structure is very common for location near something:

• у дома = by the house
• у окна = by the window
• у двери = by the door

So входа is the genitive singular of вход.

In Russian, the verb to be is usually omitted in the present tense.

So where English says:

• There is a small porch by the entrance

Russian often simply says:

• У входа небольшое крыльцо

The idea of is / there is is understood automatically.

If you wanted the past or future, Russian would use a form of быть:

• У входа было небольшое крыльцо = There was a small porch by the entrance
• У входа будет небольшое крыльцо = There will be a small porch by the entrance

Because крыльцо is a neuter noun, and the adjective has to agree with it.

So:

• небольшой = masculine
• небольшая = feminine
• небольшое = neuter
• небольшие = plural

Since крыльцо is neuter singular, the correct form is:

• небольшое крыльцо

This is standard adjective-noun agreement in Russian.

Крыльцо usually means a porch, front steps, or a small entrance platform outside a building, especially at the main door.

It is a neuter noun:

• singular: крыльцо
• prepositional singular: на крыльце
• plural: крыльца

In this sentence, it refers to the place near the entrance where the cat sits.

На этом крыльце means on this porch.

Here, на expresses location, so it takes the prepositional case:

• крыльцо → на крыльце

The adjective also changes to match:

• этот → этом

So:

• на этом крыльце = on this porch

Compare:

• на столе = on the table
• на улице = on the street
• на крыльце = on the porch

Because this must agree with the noun in gender, number, and case.

The noun is крыльце, which is:

• neuter
• singular
• prepositional case after на (for location)

So the demonstrative этот changes like this:

• nominative neuter: это
• prepositional neuter: этом

That is why:

• это крыльцо = this porch
• на этом крыльце = on this porch

Russian often repeats a noun where English might prefer it or a less repetitive structure.

So Russian says:

• У входа небольшое крыльцо, и на этом крыльце...

Literally, this is something like:

• By the entrance is a small porch, and on this porch...

Using на нём might also be possible in some contexts, but на этом крыльце sounds clear and natural because it points back to that exact porch and avoids ambiguity.

So the repetition is not strange in Russian; it is often stylistically normal.

Because кошка is the subject of the verb сидит.

In the second clause:

• часто сидит старая кошка = an old cat often sits

The subject of a verb in Russian is usually in the nominative case, so:

• кошка = nominative singular

The adjective agrees with it:

• старая кошка = old cat

Because кошка is a feminine noun, and the adjective must match it.

So:

• старый = masculine
• старая = feminine
• старое = neuter
• старые = plural

Since кошка is feminine singular, the correct form is:

• старая кошка

Часто means often. It is an adverb, so it describes the verb:

• часто сидит = often sits

Its position is fairly flexible in Russian. For example, these are all possible depending on emphasis:

• на этом крыльце часто сидит старая кошка
• старая кошка часто сидит на этом крыльце
• часто на этом крыльце сидит старая кошка

The version in your sentence sounds natural and keeps the focus on the place first, then the action, then the cat.

Russian word order is more flexible than English because grammatical endings show the relationships between words.

English relies heavily on word order:

• The old cat often sits on this porch

Russian can move parts around for emphasis, topic, or style:

• На этом крыльце часто сидит старая кошка
• Старая кошка часто сидит на этом крыльце
• Часто старая кошка сидит на этом крыльце

All of these are understandable, but they do not feel exactly the same. In your sentence, the speaker first introduces the porch, then says what often happens there.

There is a comma because и connects two clauses:

1. У входа небольшое крыльцо
2. на этом крыльце часто сидит старая кошка

Even though the first clause has no expressed verb, it still functions as a full clause in Russian. So the comma is standard punctuation here.

Yes, около входа is possible and would mean something like near the entrance.

But there is a small nuance:

• у входа = by the entrance / right at the entrance
• около входа = near the entrance

У often feels a bit closer or more directly attached to the place. In this sentence, у входа sounds very natural.

Because the subject is singular:

• кошка = cat (singular)

So the verb must also be singular:

• сидит = sits / is sitting

If the subject were plural, the verb would change:

• на этом крыльце часто сидят старые кошки = old cats often sit on this porch

It can mean either, depending on context.

Russian present-tense verbs often cover both ideas:

• сидит = sits / is sitting

In this sentence, because of часто, the meaning is habitual:

• often sits

So here it is not about one single moment, but about something that happens regularly.