Нерест начался ночью.

There isn’t one. Russian has no articles, so there is no separate word for the or a/an.

So Нерест начался ночью can mean something like the spawning began at night or spawning began at night, depending on context. Russian relies on context much more than English for definiteness.

Нерест is in the nominative case because it is the subject of the sentence — it is the thing that began.

You can think of the structure like this:

• нерест = subject
• начался = began
• ночью = at night / during the night

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

Since нерест is a masculine singular noun, the verb must also be masculine singular:

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

So начался is used because нерест is grammatically masculine.

The verb here is начаться, not начать.

• начать = to begin something, to start something
• начаться = to begin, to start

In this sentence, the spawning itself begins, so Russian uses начаться.

Compare:

• Они начали нерест = They began the spawning
• Нерест начался = The spawning began

The -ся form is very common when something begins on its own or when no outside doer is mentioned.

This is an aspect question.

• начаться → perfective
• начинаться → imperfective

Начался is perfective, so it presents the beginning as a single completed event: the moment when it started.

That fits this sentence well: it tells us when the spawning started.

If you said начинался, it would sound more like background description, repeated action, or focus on the process of beginning rather than the fact that the start happened.

So:

• Нерест начался ночью = The spawning began at night
• Нерест начинался ночью = The spawning used to begin at night / was beginning at night, depending on context

Russian often uses the instrumental case for certain time expressions, especially with parts of the day.

So:

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

Here, ночью is the instrumental form of ночь, but in this kind of expression it functions almost like an adverb of time.

Usually, no — not for the general meaning at night.

For general time reference, Russian normally says:

• ночью = at night / during the night

В ночь is used in more specific expressions, often meaning something like on the night of or into the night, for example:

• в ночь на понедельник = on the night before Monday / Sunday night into Monday

So in your sentence, ночью is the natural choice.

Yes. Russian word order is fairly flexible because the cases show grammatical roles.

The sentence Нерест начался ночью is a neutral, straightforward order.

You could also say:

• Ночью начался нерест

That still means the same basic thing, but the emphasis shifts slightly. Starting with ночью puts more focus on when it happened.

So the original version is neutral, while a reordered version can highlight a different part of the message.

It can mean either, depending on context.

Ночью is often used for a general nighttime time frame, so in English it may be translated as:

• at night
• during the night
• sometimes overnight, depending on context

In this sentence, English could choose either at night or during the night, depending on how natural the translation needs to sound.

Yes, absolutely. It is a perfectly normal complete Russian sentence.

It has everything it needs:

• Нерест = subject
• начался = finite verb
• ночью = time expression

Russian often expresses ideas more compactly than English, especially because it does not need articles and because case endings carry a lot of information.