Learn Hardcore Russian: Закрой окно, иначе будет холодно. - Close the window, otherwise it will be cold.

Закрой is the second-person singular informal imperative form of the perfective verb закрыть, meaning “close”. It expresses a direct command addressed to ты (“you”).

Why use the perfective aspect закрой instead of the imperfective закрывай?

The perfective aspect (закрыть) indicates that the action is to be completed (i.e. the window ends up closed). The imperfective (закрывать) focuses on the process (“keep closing” or “be in the act of closing”) and isn’t typical for a simple one-off command.

Why doesn’t окно change its ending in the accusative?

Окно is a neuter, inanimate noun in Russian. For neuter inanimate nouns, the nominative and accusative singular forms are identical, so you see no ending change.

What does иначе mean, and how is it used here?

Иначе means “otherwise” or “or else.” It links the first clause (закрой окно) to its logical consequence (будет холодно) if the command isn’t obeyed.

Why is there no explicit subject before будет холодно?

Russian often uses impersonal constructions for weather or sensations. In будет холодно, холодно is a predicate adjective and будет marks the future tense. English would say “it will be cold,” but Russian drops the “it.”

Can I make this command more polite or formal?

Yes. Use the formal/plural imperative закройте instead of закрой:
Закройте окно, иначе будет холодно.

Is the comma before иначе mandatory, and can I change the word order?

• Yes, you separate the clauses with a comma before иначе.
• Russian word order is fairly flexible. For example:
• Иначе будет холодно – закрой окно.
• Закрой окно, иначе будет холодно.
Both convey the same meaning; the comma and context keep it clear.