На улице дует сильный ветер, поэтому я закрываю окно.

На улице is a very common Russian expression meaning outside, outdoors, or literally in the street.

• улице is the prepositional singular form of улица.
• After на when talking about location, Russian usually uses the prepositional case.

So:

• улица = street
• на улице = on the street / outside

In this sentence, На улице is best understood as outside.

Russian often describes wind with the verb дуть, which means to blow.

So:

• дует ветер = the wind is blowing

This is a normal, natural way to say it in Russian. English often prefers It’s windy or A strong wind is blowing, but Russian commonly uses the actual subject ветер with the verb дуть.

Because сильный has to agree with ветер.

• ветер is masculine
• it is singular
• here it is in the nominative case (the subject of the clause)

So the adjective must also be:

• masculine
• singular
• nominative

That gives сильный.

Russian word order is more flexible than English word order.

Both of these are possible:

• На улице дует сильный ветер
• На улице сильный ветер дует

But На улице дует сильный ветер sounds very natural and smooth. It presents the situation almost like: Outside, a strong wind is blowing.

Starting with На улице sets the scene first, and then дует сильный ветер tells what is happening there.

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

In this sentence:

• На улице дует сильный ветер, поэтому я закрываю окно.
• A strong wind is blowing outside, so I’m closing the window.

It introduces the result or consequence of the first part.

Because поэтому connects the first clause with the result clause, and in standard Russian punctuation it is normally preceded by a comma in a sentence like this.

So the structure is:

• cause: На улице дует сильный ветер
• result: поэтому я закрываю окно

That is why the comma appears before поэтому.

Закрываю is the imperfective form, while закрою is the perfective future form.

Here, закрываю suggests:

• I am closing the window
• or I close the window in response to this situation

It focuses on the action as a process or as something happening now.

Compare:

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

So the sentence uses закрываю because it presents the action as currently happening or as an immediate reaction in progress.

In Russian, the present tense often covers both:

• the English simple present
• and the English present continuous

So:

• я закрываю окно can mean I close the window or I am closing the window

The exact meaning depends on context. In this sentence, because of the situation described just before it, the natural interpretation is I’m closing the window.

Окно is in the accusative singular because it is the direct object of закрываю.

You are closing what?
→ окно

But окно is a neuter inanimate noun, and for many such nouns, the accusative singular looks exactly the same as the nominative singular.

So:

• nominative: окно
• accusative: окно

That is why the form does not visibly change.

Not always. Russian often omits subject pronouns when they are clear from the verb ending.

So закрываю already tells us I am closing, because the ending -ю marks 1st person singular.

That means this is also possible:

• На улице дует сильный ветер, поэтому закрываю окно.

However, я is still perfectly natural. It may be included for:

• clarity
• emphasis
• contrast
• a more explicit, learner-friendly style

Russian has no articles like English a/an and the.

So:

• ветер can mean a wind or the wind
• окно can mean a window or the window

The context tells you which is meant. In this sentence, English naturally uses:

• a strong wind
• the window

But Russian does not need separate article words to express that.

Yes. Literally it means A strong wind is blowing outside, but in natural English the idea is often simply:

• It’s very windy outside

Russian often uses a more concrete structure with ветер and дует, while English may prefer a weather expression like It’s windy. Both communicate the same basic idea.