Ночью я видел, как молния освещает облака над морем.

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Breakdown of Ночью я видел, как молния освещает облака над морем.

• море: the sea
• как: how
• облако: the cloud

Questions & Answers about Ночью я видел, как молния освещает облака над морем.

Why is it «Ночью» and not «в ночь» or «в ночью»?

«Ночью» here is an adverbial form meaning “at night / during the night.”

• It comes from the noun ночь in the instrumental case and is very common in time expressions:
  • днём – in the daytime
  • утром – in the morning
  • вечером – in the evening
  • ночью – at night

«В ночью» is simply wrong; you cannot use в with ночью.

«В ночь» is possible but has a different, more literary or specific meaning, like “into the night” or “towards (that) night”. In your sentence, the neutral, most natural choice is «Ночью…» = “At night, I saw…”

Could I say «Я видел ночью, как…» instead of putting «Ночью» at the beginning? Does the word order matter?

Yes, you can say:

• Ночью я видел, как…
• Я видел ночью, как…

Both are grammatically correct.

The difference is mostly one of emphasis and style:

• «Ночью я видел…» – puts more emphasis on the time; “At night I saw…”
• «Я видел ночью…» – more neutral, focusing first on “I saw” and then adding the time detail.

In everyday speech, both orders sound natural. Russian word order is flexible; many time expressions can go either at the beginning or after the verb.