За окном темно.

Окном is the instrumental singular of окно (window).

The preposition за can mean different things depending on the case:

• with the accusative: movement to a position behind/beyond
• with the instrumental: location behind/beyond

Here, there is no movement, just location, so Russian uses за окном = outside the window / beyond the window.

In this sentence, за окном is a very natural Russian way to say outside.

It can, but за окном is a very common and idiomatic expression.

Literally, it means something like:

• behind the window
• outside the window
• beyond the window

In English we would usually just say It’s dark outside, but Russian often expresses this as За окном темно.

A similar Russian sentence is:

• На улице темно = It’s dark outside / in the street

Both are natural, but за окном gives the image of what things look like from indoors.

In the present tense, Russian often leaves out быть (to be).

So:

• За окном темно literally looks like Outside the window dark
• but it means It is dark outside

This is completely normal in Russian.

Compare:

• Он дома = He is at home
• Сегодня холодно = It is cold today
• Здесь тихо = It is quiet here

If you wanted past or future, a form of быть would appear:

• За окном было темно = It was dark outside
• За окном будет темно = It will be dark outside

Темно here is a predicative adverb/state word, not a regular adjective describing a noun.

It means:

• dark
• it is dark

Compare:

• тёмный = an adjective, used with a noun
  
  • тёмная комната = a dark room
• темно = used in sentences about a general state or condition
  
  • В комнате темно = It is dark in the room

So in За окном темно, темно does not describe a noun directly. It describes the overall situation.

In Russian, ё is often written as е in ordinary text.

So:

• тёмный may appear as темный
• тёмно may appear as темно

But the pronunciation still normally has yo:

• тёмно

This is very common in Russian writing. Learners should get used to the fact that е sometimes really means ё.

Yes, Темно за окном is also possible.

Russian word order is flexible, but it changes the emphasis a little.

• За окном темно = starts with the setting: Outside, it’s dark
• Темно за окном = starts with the condition: It’s dark outside

Both are understandable and natural in the right context.
The version За окном темно is very common and feels neutral.

Yes. Literally, за окном means behind the window or beyond the window.

In real usage, though, when talking about weather, light, or what is happening outdoors, it usually means outside.

For example:

• За окном дождь = It’s raining outside / literally There is rain outside the window
• За окном снег = There is snow outside

So the exact English translation depends on context, but outside is often the most natural one.

Yes.

These are all possible, but they are slightly different in feel:

• За окном темно = It’s dark outside, with the perspective of someone indoors looking out
• На улице темно = It’s dark outside / in the street
• На дворе темно = also It’s dark outside, but this can sound a bit more literary, old-fashioned, or regionally flavored depending on context

So За окном темно is especially natural when the speaker is inside a building.

Yes, in English we need the dummy subject it:

• It is dark outside

Russian does not need that kind of subject here.
The sentence is complete without any word for it.

That is very common with weather, time, and general conditions:

• Холодно = It is cold
• Поздно = It is late
• Светло = It is light/bright

So За окном темно is a normal subjectless Russian sentence.

A careful pronunciation is roughly:

• За окном темно
• za ak-NOM teem-NO

A few helpful points:

• о in unstressed syllables is often pronounced more like a or a reduced vowel
• the stress is:
  • окнóм
  • темнó (really тёмно, even if written темно)

So natural pronunciation is close to:

• za ak-NOM tyom-NO

You would add forms of быть:

• За окном было темно = It was dark outside
• За окном будет темно = It will be dark outside

The structure stays the same:

• location phrase: за окном
• state word: темно
• verb быть appears only when needed for tense

That is a very useful pattern:

• В комнате было тихо = It was quiet in the room
• Утром будет холодно = It will be cold in the morning