Ночью у меня начался кашель.

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.

In this sentence, у меня is a very natural Russian way to express that something happened to me or I experienced something.

Literally, у меня means something like by me / at my place / with me, but in real usage it often means:

• I have
• something happened to me
• I experienced

So:

• У меня начался кашель = literally A cough started with me / by me
• Natural English: I developed a cough or I started having a cough

Russian often uses this structure where English would use I as the subject.

Compare:

• У меня болит голова. = My head hurts / I have a headache.
• У меня поднялась температура. = My temperature went up / I got a fever.

So у меня is not strange here at all—it is the normal idiomatic pattern.

Because кашель is the grammatical subject of the verb начался.

In Russian, the thing that begins is usually in the nominative:

• начался дождь = the rain started
• началась гроза = the storm started
• начался кашель = the cough started

So even though English often says I started coughing, Russian here is structured more like:

• A cough started

That is why кашель stays in the nominative singular.

Because Russian past tense verbs agree with the subject in gender and number.

The subject here is кашель, which is a masculine singular noun, so the verb must also be masculine singular in the past tense:

• начался — masculine singular
• началась — feminine singular
• началось — neuter singular
• начались — plural

Examples:

• начался кашель = cough started
• началась боль = pain started
• началось воспаление = inflammation started
• начались симптомы = symptoms started

So начался is masculine because кашель is masculine.

Here -ся is part of the verb начаться, which means to begin / to start in the sense of something beginning on its own.

Compare:

• начать = to start something
• начаться = to begin / to start (for the event itself)

Examples:

• Я начал урок. = I started the lesson.
  (начал = I started something)

• Урок начался. = The lesson began.
  (начался = the lesson itself began)

In your sentence:

• кашель начался = the cough began

So -ся does not mean a literal reflexive idea like myself here. It is just part of the verb form meaning to begin intransitively.

Because начался is perfective, and here Russian is talking about the start of the cough as a completed event.

• начаться → perfective
• начинаться → imperfective

With начался, the speaker is saying that at some point during the night, the cough started.

That fits the sentence well because the focus is on the moment of onset.

By contrast, начинался would usually sound different in meaning or context, for example if you were describing an ongoing situation, background, or repeated pattern. In this sentence, Russian normally prefers начался.

So:

• Ночью у меня начался кашель. = During the night, a cough started / I developed a cough.

The perfective verb is the natural choice.

Both can be translated naturally in English, but they are not exactly the same.

Ночью у меня начался кашель

This focuses on the condition/symptom appearing.

It suggests:

• I developed a cough
• A coughing symptom began

This can sound a bit more medical or descriptive.

Ночью я начал кашлять

This focuses on the action of coughing.

It suggests:

• I started coughing at night

So the difference is roughly:

• начался кашель = the symptom began
• начал кашлять = I began the action of coughing

Both are correct, but the sentence you gave is especially natural when describing the onset of an illness or symptom.

Yes, Russian word order is fairly flexible, and У меня ночью начался кашель is also correct.

The original sentence:

• Ночью у меня начался кашель

puts Ночью first, so the time frame is emphasized first: At night...

Other possible orders include:

• У меня ночью начался кашель.
• Кашель у меня начался ночью.
• Ночью начался у меня кашель. (possible, but less neutral)

The basic meaning stays the same, but the focus changes.

Very roughly:

• Ночью у меня начался кашель = emphasizes when
• У меня ночью начался кашель = emphasizes with me / in my case
• Кашель у меня начался ночью = emphasizes the cough

So the original order is natural and neutral, especially in storytelling or describing symptoms.

Russian has no articles, so there is no separate word for a, an, or the.

That means кашель can mean:

• a cough
• the cough
• just cough in a general sense

English has to choose an article, but Russian does not.

You understand whether it is a or the from context. In this sentence, English would usually say a cough:

• At night I developed a cough.

So nothing is missing in Russian—Russian simply does not use articles.

The stress is:

• НОчью
• у менЯ
• началсЯ
• кАшель

So the whole sentence is:

НОчью у менЯ началсЯ кАшель.

A few pronunciation notes:

• чью in ночью can be tricky; it is roughly like chyu
• у меня is usually pronounced smoothly together in speech
• in начался, the stress is on the last -ся
• кашель has stress on the first syllable: кА-

If you want a rough English-style approximation, it might sound like:

NO-chyu oo mi-NYA na-chal-SYA KA-shel

That is only approximate, but it helps locate the stress.

Yes. Depending on context, that is a very natural English rendering.

The Russian sentence literally describes the onset of the symptom, so possible English translations include:

• I developed a cough during the night.
• I came down with a cough at night.
• I started having a cough during the night.
• I started coughing during the night. (a bit less literal, but often natural)

The exact best translation depends on style:

• developed a cough = more neutral/medical
• came down with a cough = more conversational
• started coughing = focuses more on the action than the symptom

So yes, I came down with a cough is a good natural interpretation in many contexts.