Я по ошибке выключил свет.

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Breakdown of Я по ошибке выключил свет.

• свет: the light
• выключить: to turn off
• ошибка: the mistake

Questions & Answers about Я по ошибке выключил свет.

Why is the verb выключил ending in -ил?

Because this is the past tense masculine singular form of выключить.

In Russian, past tense verbs agree with the subject in gender and number:

• я выключил = I turned off / I switched off (male speaker)
• я выключила = I turned off / I switched off (female speaker)
• мы выключили = we turned off

So if the speaker is a man, выключил is correct. If the speaker is a woman, it would be Я по ошибке выключила свет.

Why is it выключил, not выключал?

Выключил is perfective, while выключал is imperfective.

Here, the sentence describes one completed action: the light ended up off. That is why Russian uses the perfective verb:

• выключил = turned off, completed the action

If you used выключал, it would usually sound like:

• a repeated action,
• a process,
• or background information rather than a single completed result.

For example:

• Я выключал свет каждый вечер. = I used to turn off the light every evening.

So in this sentence, выключил is the natural choice.

What exactly is doing here?