Один комар сел мне на руку, когда я стоял под берёзой.

Yes, один literally means one, but in a sentence like this it often works like a single or one in English:

• Один комар = one mosquito / a mosquito
• It can add a slight sense of one particular mosquito or a single mosquito

Russian has no articles (a / the), so words like один can sometimes help introduce something new into the story.

Compare:

• Комар сел мне на руку = A/The mosquito landed on my hand
• Один комар сел мне на руку = One mosquito landed on my hand

So here один is not just mathematical; it also helps set the scene.

Because комар is the subject of the sentence, so it is in the nominative case.

The mosquito is the thing doing the action:

• комар = the mosquito
• сел = sat down / landed

So:

• Один комар сел... = One mosquito landed...

If the mosquito were an object instead, its form might change. But here it is the subject, so nominative is used.

This is a very common and important question. The difference is about aspect and also about meaning.

• сесть / сел = to sit down, to settle, to land
  This is a completed action.
• сидеть / сидел = to be sitting
  This describes a state, not the moment of landing.

In this sentence, the mosquito performs a single action:

• it landed on the hand

So Russian uses сел.

Compare:

• Комар сел мне на руку = The mosquito landed on my hand
• Комар сидел у меня на руке = The mosquito was sitting on my hand

So сел focuses on the moment it came down and settled.

Because Russian past tense verbs agree with the subject in gender and number.

The subject is комар, which is a masculine singular noun. So the past tense form is:

• masculine singular: сел
• feminine singular: села
• neuter singular: село
• plural: сели

Examples:

• Комар сел = the mosquito landed
• Муха села = the fly landed
• Насекомое село = the insect landed
• Комары сели = the mosquitoes landed

Russian very often uses the dative case to show the person affected, where English uses a possessive:

• мне на руку = literally onto me on the hand
• natural English: on my hand

This structure is very common in Russian, especially with body parts.

Examples:

• Он посмотрел мне в глаза = He looked me in the eyes
• Она взяла меня за руку = She took me by the hand
• Мне больно в спину = My back hurts / It hurts in my back

So мне does not mean to me in a simple literal English way here. It marks whose hand it is.

You could say на мою руку, but мне на руку sounds more natural in this kind of sentence.

Because Russian often uses the dative to mark the person affected by an action, especially when the action involves that person's body part or personal space.

Here:

• мне = dative of я
• It means something like to me / on me
• Together with на руку, it gives the sense onto my hand

So the structure is:

• сел мне на руку = landed on my hand

This is a very typical Russian pattern and is worth remembering as a chunk.

Because на can take different cases depending on whether there is movement or location.

Here the mosquito moves onto the hand, so Russian uses:

• на + accusative for motion toward / onto
• на + prepositional for location on

So:

• сел на руку = landed onto the hand → motion → accusative
• сидел на руке = was sitting on the hand → location → prepositional

This is the same pattern as:

• положить на стол = put onto the table
• лежать на столе = lie on the table

So руку is accusative because the verb describes movement onto the hand.

Yes, рука is feminine, and руку is exactly the accusative singular form of рука.

Declension:

• nominative: рука
• accusative: руку
• prepositional: руке
• instrumental: рукой / рукою

Since на here expresses motion onto something, it requires the accusative:

• на руку = onto the hand

So the feminine noun changes from рука to руку.

Because стоять and встать mean different things, and aspect matters again.

• стоять / стоял = to be standing
• встать / встал = to stand up, to get up

The sentence describes the background situation at the time the mosquito landed:

• когда я стоял под берёзой = when I was standing under a birch tree

This is an ongoing state, so Russian uses the imperfective verb стоять.

If you said когда я встал, that would mean:

• when I stood up / when I got up

That would describe a completed action, not the ongoing position.

Because the clause after когда gives the background scene during which the main event happened.

The main event is:

• Один комар сел мне на руку
  a single completed event

The background is:

• когда я стоял под берёзой
  an ongoing situation

Russian commonly uses:

• perfective for the main completed event
• imperfective for the ongoing background

So:

• сел = the mosquito landed
• стоял = I was standing

This contrast is very natural in Russian narration.

Because the preposition под can take different cases depending on meaning.

For location (under something), Russian uses:

• под + instrumental

For movement to a position under something, Russian usually uses:

• под + accusative

In this sentence, the speaker is already standing there, so it is location:

• под берёзой = under a birch tree

Compare:

• Я стоял под берёзой = I was standing under a birch tree
• Я встал под берёзу = I moved to a position under the birch tree

So берёзой is instrumental because it expresses location with под.

Берёзой is the instrumental singular form of берёза.

Basic form:

• берёза = birch tree

Case forms:

• nominative: берёза
• accusative: берёзу
• instrumental: берёзой

Since the sentence uses под in the sense of under as a location, instrumental is required:

• под берёзой = under a birch tree

Yes. The letter ё is always stressed, so:

• берёза
• берёзой

This helps with pronunciation.

In printed Russian, ё is often replaced by е, so you may see:

• березой

But it is still pronounced berYOzoy, not berezoy with a plain unstressed e sound.

For learners, it is useful to remember the real spelling and pronunciation with ё.

Because когда я стоял под берёзой is a subordinate clause.

Russian separates subordinate clauses with commas, including clauses introduced by когда (when).

So the sentence has:

• main clause: Один комар сел мне на руку
• subordinate clause: когда я стоял под берёзой

That is why there is a comma.

Yes. Russian word order is relatively flexible, because case endings show grammatical roles.

The given order is natural:

• Один комар сел мне на руку, когда я стоял под берёзой.

But other orders are possible, with different emphasis. For example:

• Когда я стоял под берёзой, один комар сел мне на руку.
  This puts the time/background first.
• Мне на руку сел один комар...
  This emphasizes onto my hand.
• На руку мне сел один комар...
  Also possible, with stronger focus on where it landed.

The original sentence is a neutral, clear narrative order.

In this context, landed is the most natural English translation.

Literally, сесть means to sit down, but Russian uses it much more broadly for insects and birds coming to rest somewhere:

• Комар сел на руку = A mosquito landed on the hand
• Птица села на ветку = A bird landed on a branch

So while the core idea is settled into a sitting position, the best English wording here is usually landed.

Because Russian has no articles like a or the.

Whether something is understood as definite or indefinite depends on context, word order, and sometimes words like один, этот, etc.

So:

• комар can mean a mosquito or the mosquito
• под берёзой can mean under a birch tree or under the birch tree

In this sentence:

• Один комар strongly suggests one / a mosquito
• под берёзой is naturally understood as under a birch tree, unless the context has already identified a specific birch

So Russian leaves that distinction to context rather than articles.