Правонарушение произошло ночью.

Because the subject, правонарушение, is a neuter singular noun.

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

• masculine: произошёл
• feminine: произошла
• neuter: произошло
• plural: произошли

So:

• правонарушение произошло
• авария произошла
• инцидент произошёл

It is in the nominative case because it is the subject of the sentence — the thing that occurred.

So in Правонарушение произошло ночью, the structure is basically:

• Правонарушение = the offense/violation
• произошло = occurred
• ночью = at night

Ночью is the instrumental singular of ночь.

Russian often uses certain nouns in the instrumental case as adverbial expressions of time, especially parts of the day:

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

So ночью does not literally mean by night in normal modern usage. It simply means at night or during the night.

Because ночью is the normal way to say at night.

The alternatives mean different things or sound unnatural here:

• ночью = at night / during the night
• в ночь = on the night of, usually with a reference point, as in в ночь на понедельник = on the night before Monday
• в ночи is not the normal way to say at night in standard everyday Russian

So for a general statement like this one, ночью is exactly what you want.

Произошло is the past tense of the perfective verb произойти.

The perfective aspect is used because the sentence presents the event as a completed fact: the offense occurred.

Compare:

• Правонарушение произошло ночью. = The offense occurred at night.
  Completed event, simple fact.

• Правонарушение происходило ночью. = The offense was happening at night / used to happen at night.
  This sounds different and would only fit a special context.

In a straightforward report of an event, произошло is the natural choice.

Both can often be translated as happened or occurred, but произошло is more neutral and formal here.

• произойти is common in reports, official language, and factual descriptions
• случиться is often a little more conversational or general

With something like правонарушение, произошло sounds especially natural because the whole sentence has a formal, report-like tone.

Not exactly.

• правонарушение = violation, offense, legal wrongdoing in a broad sense
• преступление = crime

So правонарушение is a broader and often more technical/legal term. Not every правонарушение is necessarily a serious преступление.

An English speaker might be tempted to treat them as exact synonyms, but they are not.

Yes. Russian word order is flexible, although the emphasis changes.

The neutral order here is:

• Правонарушение произошло ночью.

You could also say:

• Ночью произошло правонарушение.

This version puts more focus on when it happened.

In English, word order is much more rigid. In Russian, changing the order often changes focus rather than basic meaning.

The stress is:

• правонаруше́ние
• произошло́
• но́чью

A rough pronunciation guide:

• пра-ва-на-ру-ШЕ-ни-ye
• пра-и-за-шЛО
• НО-чью

The most important thing is to place the stress correctly, especially in произошло́ and но́чью.

Because Russian has no articles.

So Правонарушение произошло ночью can mean:

• The offense occurred at night
• An offense occurred at night

Which one is intended depends on context.

Russian usually expresses definiteness through context, word order, and the situation, not with separate words like the or a.