Маленький воробей сидел у порога.

Because it is agreeing with воробей, which is:

• masculine
• singular
• nominative case

Russian adjectives change their endings to match the noun they describe. Here the basic agreement is:

• маленький воробей = masculine singular nominative

So маленький is the correct form for describing воробей as the subject of the sentence.

Воробей is in the nominative singular, because it is the subject of the sentence — the one doing the action of сидел.

In this sentence:

• воробей = the sparrow
• сидел = was sitting / sat

A good beginner rule is: the noun that is the subject is often in the nominative.

Also, воробей is the dictionary form of the noun.

Сидел is the past tense, masculine singular form of сидеть.

Russian past tense works differently from English:

• it agrees with the subject in gender and number
• it does not change for person the way present tense does

So:

• он сидел = he was sitting / he sat
• она сидела = she was sitting
• оно сидело = it was sitting
• они сидели = they were sitting

Since воробей is a masculine singular noun, the verb becomes сидел.

Russian has no articles like English the and a/an.

So маленький воробей can mean:

• a small sparrow
• the small sparrow

Which one is meant depends on the context, not on a separate article.

That is very normal in Russian, and learners usually just get used to understanding it from context.

After the preposition у, Russian normally uses the genitive case.

The noun here is:

• dictionary form: порог = threshold, doorstep

After у, it becomes:

• у порога

So порога is genitive singular.

This is one of the most important things to learn about Russian prepositions: many of them require a specific case, and у usually takes the genitive.

Here у means something like:

• by
• near
• at

So у порога gives the idea of being by the threshold / near the doorstep.

It does not literally mean being directly on top of it. It usually suggests position close to something.

This is why у is often used for location near a person or object:

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

Yes, but the meaning changes a bit.

• у порога = by the threshold, near the doorstep
• на пороге = on the threshold / at the doorway itself

So на пороге sounds more physically on the threshold, while у порога is a little broader and more like near the threshold.

Both can be natural depending on the exact situation.

Yes. In neutral Russian, adjectives usually come before the noun:

• маленький воробей

That is the most standard order.

Russian word order is more flexible than English, so you can move things around for emphasis or style. But for ordinary, neutral description, adjective + noun is the safest pattern.

No, Russian word order is fairly flexible.

This sentence has a very neutral order:

• subject + verb + location

But other orders are possible, for example:

• У порога сидел маленький воробей.
• Сидел у порога маленький воробей.

These can shift emphasis:

• starting with у порога highlights the location
• starting with сидел can sound more literary or descriptive

Even though the order changes, the case endings still show who is doing what.

It can mean either, depending on context.

Because сидеть is imperfective, сидел usually describes:

• a state
• an ongoing situation
• background description

So in English it may be translated as:

• was sitting
• sometimes simply sat

In a descriptive sentence like this, was sitting is often the closest feel.

The dictionary form is сидеть.

Russian dictionaries usually list verbs in the infinitive, just like English dictionaries use to sit, to read, to go, etc.

In the sentence, you see the verb in a conjugated form:

• сидеть → сидел

That change tells you grammatical information:

• past tense
• masculine
• singular

So the sentence uses the form needed for this subject, not the dictionary form.

The stress is:

• воробе́й
• поро́га

That matters a lot in Russian.

A rough guide:

• воробе́й ≈ va-ra-BYEY
• поро́га ≈ pa-RO-ga

Notice that unstressed о is often pronounced closer to a than to a clear English o sound. So learners often hear:

• воробей sounding closer to варабей
• порога sounding closer to парога

That is normal Russian vowel reduction.

Because the subject is already named: маленький воробей.

Russian often leaves out pronouns when they are unnecessary. Since the noun itself is present, there is no need to add он.

So instead of saying something like The small sparrow, he was sitting..., Russian simply says:

• Маленький воробей сидел у порога.

That is completely natural.

Воробей is masculine.

You can tell this in a few ways:

1. Its past-tense verb is сидел, the masculine form.
2. The adjective is маленький, also masculine.
3. Many nouns referring to animals can have grammatical gender independent of real biological sex, and воробей is grammatically masculine.

So even if the sentence is just about a sparrow in general, Russian grammar treats воробей as masculine.

Yes, completely.

It has everything a basic Russian sentence needs:

• subject: маленький воробей
• verb: сидел
• location phrase: у порога

Russian often expresses ideas very compactly. Short sentences like this are very common and perfectly natural.