Ночью у меня начался кашель.

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Breakdown of Ночью у меня начался кашель.

• начаться: to start
• кашель: the cough
• ночью: during the night

Questions & Answers about Ночью у меня начался кашель.

Why is ночью used here, and what case is it?

Ночью is the instrumental case of ночь (night), and here it is being used adverbially to mean at night / during the night.

This is a very common Russian pattern with words for times of day:

• утром = in the morning
• днём = in the daytime / during the day
• вечером = in the evening
• ночью = at night

So in this sentence, Ночью tells you when the cough began.

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Why does Russian say у меня instead of just using я?