Кто-то тихо стучит в дверь, а я ещё не сплю.

Кто-то means someone.

It is made from:

• кто = who
• -то = a particle that makes it indefinite, so who becomes someone

So Кто-то тихо стучит в дверь means Someone is quietly knocking on/at the door.

A learner may also notice similar patterns:

• что-то = something
• где-то = somewhere
• когда-то = sometime

In this sentence, кто-то is the subject.

Стучит is the 3rd person singular present-tense form of стучать.

That matches кто-то, because кто-то is grammatically singular: someone knocks / is knocking.

The verb forms are:

• я стучу = I knock
• ты стучишь = you knock
• он / она стучит = he / she knocks
• кто-то стучит = someone knocks

Russian often uses the present tense where English might use either knocks or is knocking, depending on context. Here, English would usually say is knocking.

Because тихо is an adverb, and it describes how the action happens.

• тихий = quiet, soft, calm (adjective)
• тихо = quietly, softly (adverb)

Since it modifies the verb стучит (is knocking), Russian uses the adverb:

• тихо стучит = is knocking quietly / softly

An adjective would be used with a noun, not directly with the verb.

Here в дверь is the normal expression with стучать.

The noun дверь is feminine, and in this sentence it is in the accusative singular:

• nominative: дверь
• accusative: дверь

For this noun, nominative and accusative singular look the same.

The phrase стучать в дверь means to knock on/at the door.

This may feel strange because в often means in / into, but with some verbs Russian uses fixed expressions that do not match English word-for-word. So it is best to learn стучать в дверь as a set phrase.

А often connects two clauses with a sense of contrast, switch of focus, or meanwhile, while и is more simply and.

So:

• ..., а я ещё не сплю. = ..., while I’m still awake / but I’m still not asleep.

Here а works well because the sentence shifts from someone knocking to my state. There is a mild contrast or change of viewpoint.

If you used и, it would sound more like simple addition. А feels more natural here.

In this sentence, ещё не сплю most naturally means:

• I’m still awake
• I’m not asleep yet

Word by word:

• ещё = still / yet
• не = not
• сплю = I sleep / am sleeping

Russian expresses this idea with the present tense verb спать. English usually prefers I’m not asleep yet or I’m still awake, rather than the more literal I’m still not sleeping.

So the Russian is natural, even if the most natural English translation is less literal.

Because сплю describes the speaker’s current state in the present:

• я не сплю = I’m not sleeping / I’m awake

Forms like уснул or заснул refer to falling asleep, usually as a completed event in the past:

• я уснул = I fell asleep
• я не уснул = I didn’t fall asleep

So я ещё не сплю is about right now: the speaker has not gone to sleep yet.

Yes, Russian word order is flexible, but the version here is the most neutral.

Current order:

• Кто-то = subject first
• тихо = adverb
• стучит = verb
• в дверь = destination/object of the knocking

This gives a natural, neutral flow.

Other orders are possible, but they change emphasis:

• Тихо кто-то стучит в дверь = emphasizes quietly
• В дверь кто-то тихо стучит = emphasizes at the door
• Кто-то стучит в дверь тихо = possible, but less neutral

For learners, the original word order is a good default.

A couple of points often help learners:

кто-то

• The к in кто is pronounced; it is not silent like the k in English knight.
• So it sounds roughly like kto-ta.

ещё

• The letter ё is always stressed and sounds like yo.
• So ещё is pronounced roughly ish-YO or yesh-CHO depending on how detailed you want to be about the consonants, but the key point is: stress is on ё.

Also:

• стучит is stressed on the last syllable: стучи́т
• дверь is one syllable
• сплю is also one syllable

Yes. Стучать is the imperfective verb.

That fits because the sentence describes an ongoing action happening right now:

• Кто-то тихо стучит в дверь = someone is knocking

Russian commonly uses the imperfective for actions in progress or repeated actions.

A related perfective verb is постучать, but that would usually be used in a different structure or context, for example when talking about a completed act of knocking:

• Кто-то постучал в дверь = someone knocked on the door

Here the speaker is describing what is happening at the moment, so стучит is exactly what you would expect.

Russian often expresses being awake by simply saying not sleeping:

• Я не сплю = I’m awake

English often prefers a separate adjective or expression:

• I’m awake
• I’m not asleep

Russian can also use бодрствовать, but that is more formal or specialized and would sound unnatural in an everyday sentence like this.

So я ещё не сплю is the normal everyday way to say that the speaker is still awake.

Yes, depending on intonation and context.

The grammar itself is neutral, but the sentence can suggest different feelings:

• simple observation: someone is knocking, and I’m still awake
• mild surprise: someone is knocking even though it’s late
• mild annoyance: someone is knocking, and I still haven’t gone to sleep

The phrase ещё не сплю often hints that the speaker has not gone to bed yet or has not managed to fall asleep yet, which can add a subtle emotional tone depending on context.