Breakdown of Вчера мы установили датчик, как бы кто-нибудь не отключил сигнализацию.
не
not
мы
we
вчера
yesterday
кто-нибудь
anyone / someone (unspecified)
бы
would (conditional particle) / concessive particle
установить
to install
отключить
to cut off
как бы не
lest (in 'как бы не')
датчик
sensor
сигнализация
alarm system
Questions & Answers about Вчера мы установили датчик, как бы кто-нибудь не отключил сигнализацию.
Why is the verb отключил in the past tense if the action of turning off the alarm hasn't happened yet?
In Russian, clauses of apprehension (worrying that something might happen) use как бы не paired with a past tense verb. Even though the fear is about a potential future event, this structure grammatically requires the past tense, much like how the particle бы is generally followed by the past tense in conditional sentences.
Does the не in не отключил mean the sentence is about someone not turning off the alarm?
No, it means the exact opposite. In this specific structure, как бы не acts as a fixed phrase meaning 'lest' or 'for fear that.' The не does not negate the verb; instead, the whole construction expresses a worry that the action will happen.
Why use кто-нибудь here instead of кто-то?
кто-то refers to a specific, albeit unknown, person. кто-нибудь refers to absolutely anyone, which fits perfectly here because the threat is a hypothetical, undefined future possibility. You are afraid that anyone at all might turn off the alarm.
Why is отключил perfective rather than imperfective?
The speaker is worried about the successful, completed result of the action, which is the alarm actually being disabled. You use the perfective aspect here because the fear is focused on that final result, not the ongoing process of someone trying to turn it off.
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