Утром на подоконник сел воробей.

Утром is the instrumental singular form of утро (morning).

In Russian, some time expressions use the instrumental case in an adverb-like way. So:

• утром = in the morning
• вечером = in the evening
• днём = during the day / in the daytime

So even though утро is a noun, утром functions almost like an adverb here.

Because this sentence describes movement toward a place, not being located there already.

With на:

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

So:

• сел на подоконник = sat down onto / landed on the windowsill
• сидел на подоконнике = was sitting on the windowsill

Here the sparrow moves to the windowsill, so Russian uses подоконник in the accusative: на подоконник.

Because сел means sat down / landed / took a seat, while сидел means was sitting.

This is the difference between an action happening and a state:

• сел = the moment of sitting down, landing, taking position
• сидел = already being in that position

So:

• Утром на подоконник сел воробей.
  = In the morning, a sparrow landed on the windowsill.
• Утром на подоконнике сидел воробей.
  = In the morning, a sparrow was sitting on the windowsill.

Сел is the masculine singular past tense form of сесть.

Breakdown:

• infinitive: сесть = to sit down
• past masculine: сел
• past feminine: села
• past neuter: село
• past plural: сели

It is masculine singular because the subject воробей is a masculine singular noun.

Russian word order is much freer than English word order. The sentence could also be arranged differently, for example:

• Воробей утром сел на подоконник.
• Утром воробей сел на подоконник.
• На подоконник утром сел воробей.

The version Утром на подоконник сел воробей sounds natural because it presents the scene first and introduces the subject at the end. This can make the sparrow feel like new information.

A rough information structure is:

• Утром = time setting
• на подоконник = place/direction
• сел = action
• воробей = who it was

So the final position helps highlight воробей.

Because it is still the subject of the sentence.

In Russian, word order does not determine the subject the way it often does in English. Case endings do that. Воробей is nominative singular, so it is the one performing the action.

Even though it comes last, it still means:

• the sparrow did the landing

not

• something happened to the sparrow

Both ideas are connected.

Literally, сел means sat down. But with birds, insects, and even airplanes, Russian often uses сесть in the sense of land or settle onto something.

So in this sentence:

• literal idea: a sparrow sat down on the windowsill
• natural English translation: a sparrow landed on the windowsill

So if you translate it as landed, that is very natural English.

Russian has no articles like a or the.

So воробей can mean:

• a sparrow
• the sparrow

The exact meaning depends on context. In this sentence, English usually says a sparrow because the bird is being introduced for the first time.

Yes, but it would mean something different.

Compare:

• Утром на подоконник сел воробей.
  A sparrow landed on / sat down onto the windowsill.

• Утром на подоконнике сидел воробей.
  A sparrow was sitting on the windowsill.

So the original sentence focuses on the change of position. The version with на подоконнике focuses on the resulting location/state.

Yes. It is a completely natural, standard Russian sentence.

It has everything it needs:

• Утром = when
• на подоконник = where to
• сел = what happened
• воробей = who did it

Russian often omits words that English might need, especially articles, and it allows flexible word order. So although it may feel compact to an English speaker, it is perfectly normal Russian.