ziko de hitori ga sibousimasita.

It should almost certainly be 一人, not 一り.

• 一人 is the standard way to write ひとり (one person).
• The second character is 人 (person), not the hiragana り.

So the natural sentence is:

事故で一人が死亡しました。

A learner might see 一り because of a typo, font issue, or mistaken transcription.

It is read ひとり.

This is one of the irregular readings for counting people:

• 一人 = ひとり
• 二人 = ふたり
• after that, the readings are more regular:
  • 三人 = さんにん
  • 四人 = よにん
  • 五人 = ごにん

So in this sentence, 一人が is read ひとりが.

In this sentence, で marks the circumstance or cause connected to the event.

So 事故で means something like:

• in an accident
• because of an accident
• due to an accident

In news-style Japanese, Nで is often used this way to show what event or cause led to the result.

Examples:

• 火事で二人がけがをしました。
  Two people were injured in a fire.
• 地震で家が倒れました。
  A house collapsed in the earthquake.

So 事故で sets the scene: the death happened as a result of an accident.

が marks 一人 as the subject of the sentence.

Here, 一人が死亡しました means one person died / one person has been confirmed dead.

Why が is natural here:

• it presents new information
• it focuses on the specific fact that one person is the one who died
• this is very common in reports and announcements

If you used は, the nuance would change. 一人は could sound more contrastive, as if you are setting up a comparison:

• 一人は死亡しましたが、もう一人は助かりました。
  One person died, but the other survived.

So in the original sentence, が is the normal choice.

死亡しました is a more formal, objective, and impersonal way to say died. It is very common in:

• news reports
• official announcements
• police or medical reports

Compare:

• 死にました = more direct, everyday verb form of to die
• 死亡しました = more formal/reporting style

Structure:

• 死亡 is a noun meaning death
• 死亡する is a suru-verb
• past polite form: 死亡しました

So this sentence sounds like news or official reporting, not casual conversation.

Yes. More precisely, it is a noun + する verb.

Breakdown:

• 死亡 = death
• する = to do
• 死亡する = to die / to pass away / to be dead in an official-report sense
• 死亡しました = past polite form

This pattern is very common in Japanese, especially in formal language.

Other examples:

• 勉強する = to study
• 出発する = to depart
• 説明する = to explain

So even though 死亡 is a noun by itself, 死亡しました functions as the main verb of the sentence.

Because Japanese normally places the main verb at the end of the sentence.

So the structure is:

• 事故で = in an accident
• 一人が = one person
• 死亡しました = died

This end-verb pattern is one of the most important features of Japanese grammar.

A natural English translation may move things around, but Japanese keeps the predicate at the end.

Yes. It sounds like news/report language.

Signs of that style:

• 死亡しました instead of the less formal 死にました
• concise wording
• focus on facts rather than personal feelings
• no extra details unless needed

This is the kind of sentence you might hear in a news report:

• 事故で三人がけがをしました。
• 火事で一人が死亡しました。

So if a learner feels this sentence sounds a bit stiff or official, that impression is correct.

No. Normal Japanese is usually written without spaces.

So the standard written form is:

事故で一人が死亡しました。

Spaces are often added only in beginner materials to make the sentence easier to read.

Yes, but that would change the nuance.

• 事故で一人が死亡しました。
  neutral statement of fact
• 事故では一人が死亡しました。
  adds contrast or topic marking, something like as for the accident, one person died or in that accident, one person died

Using では may suggest comparison with another situation or a shift in topic. In a plain report, で is more neutral and natural.

Because Japanese, especially in news-style sentences, often states only the information that matters most at that moment.

Here the sentence focuses on the result:

• there was an accident
• one person died

The identity of the person may be:

• unknown
• omitted because it is not the point of this sentence
• mentioned in a later sentence

This kind of omission is very normal in Japanese if the sentence is still clear.