Снаружи холодно, поэтому я закрываю окно.

Снаружи means outside, outdoors, or on the outside.

In this sentence, Снаружи холодно means It’s cold outside.

A useful point: снаружи is an adverb, so it does not change its form.

You may also hear на улице for outside/outdoors. In many everyday contexts, Снаружи холодно and На улице холодно are very close in meaning.

Because холодно is being used as a predicative word meaning it is cold.

• холодный = cold as an adjective describing a noun
  
  • холодный чай = cold tea
• холодно = cold as a state or condition
  
  • Здесь холодно = It is cold here

So in Снаружи холодно, Russian is not describing a noun; it is describing the situation or environment.

Russian usually does not use a dummy subject like English it in weather and general-condition sentences.

English says:

• It’s cold
• It’s raining

Russian simply says:

• Холодно
• Идёт дождь

So Снаружи холодно is completely normal even though there is no separate word for it.

Поэтому means therefore, so, or that’s why.

It introduces a result:

• Снаружи холодно, поэтому я закрываю окно.
• It’s cold outside, so I’m closing the window.

The comma is used because Russian normally separates the first clause from the result word поэтому.

A helpful comparison:

• потому что = because
• поэтому = therefore / so

So:

• Я закрываю окно, потому что снаружи холодно. = I’m closing the window because it’s cold outside.
• Снаружи холодно, поэтому я закрываю окно. = It’s cold outside, so I’m closing the window.

Закрываю is the imperfective present-tense form of закрывать.

It can mean:

• I am closing
• I close / I’m closing depending on context

Here it sounds natural because the action is being presented as something currently happening or in progress.

Compare:

• я закрываю окно = I’m closing the window / I close the window
• я закрою окно = I will close the window

So if the speaker is doing it now because it is cold, закрываю works well.

These are an aspect pair:

• закрывать = imperfective
• закрыть = perfective

Very roughly:

• imperfective focuses on the process, repetition, or general action
• perfective focuses on completion or a single finished result

Examples:

• Я закрываю окно. = I am closing the window.
• Я закрою окно. = I will close the window / I’ll shut the window.
• Я закрыл окно. = I closed the window.

This is one of the most important verb patterns in Russian.

Because окно is the direct object of закрываю, so it is in the accusative case.

However, окно is a neuter inanimate noun, and for many neuter inanimate nouns, the nominative and accusative singular forms are the same.

So:

• nominative: окно
• accusative: окно

That is why the form does not change here.

Compare with another noun type where the form does change:

• Я вижу стол = I see a table
• Я вижу машину = I see a car

Yes, absolutely.

Both are natural, but they organize the idea differently:

• Снаружи холодно, поэтому я закрываю окно.
  = It’s cold outside, so I’m closing the window.
  This starts with the situation and then gives the result.

• Я закрываю окно, потому что снаружи холодно.
  = I’m closing the window because it’s cold outside.
  This starts with the action and then gives the reason.

So the meaning is very close, but the information flow is a little different.

Yes. Russian word order is fairly flexible, although some orders sound more natural than others depending on emphasis.

The original sentence is neutral and natural:

• Снаружи холодно, поэтому я закрываю окно.

You could also say:

• Поэтому я закрываю окно.
  if the reason was already understood from context
• Я закрываю окно, потому что снаружи холодно.

But not every rearrangement sounds equally natural in an ordinary neutral sentence. Russian word order often changes to show focus, contrast, or what is already known in the conversation.

So for a learner, the original version is a very good standard model.