keisatu ga ziko ni taisyosita.

It is read:

けいさつ が じこ に たいしょした。
Keisatsu ga jiko ni taisho shita.

Word by word:

• 警察 = police
• が = subject marker
• 事故 = accident
• に = particle used with 対処する
• 対処した = dealt with / handled / responded to

が marks 警察 as the grammatical subject of the sentence.

So 警察が対処した means the police dealt with it.

In this sentence, 警察 is the thing doing the action, so が is natural.

Compare:

• 警察が事故に対処した。 = The police dealt with the accident.
• 警察は事故に対処した。 = As for the police, they dealt with the accident.

Both can be correct, but が tends to present the subject more directly, while は adds a topic nuance.

Because 対処する commonly takes に.

The pattern is:

• X に 対処する = to deal with X, to handle X, to address X

So:

• 事故に対処する = to deal with an accident
• 問題に対処する = to deal with a problem
• 危機に対処する = to cope with a crisis

Even though English often uses a direct object, Japanese uses に with this verb.

Because 対処する normally does not take を for the thing being dealt with.

So:

• 事故に対処した = correct
• 事故を対処した = unnatural / incorrect in standard Japanese

This is just a property of the verb. In Japanese, different verbs require different particles, and you have to learn them together with the verb.

A good way to memorize it is as a set:

• 〜に対処する

not

• 〜を対処する

対処した is the past plain form of 対処する.

Breakdown:

• 対処する = to deal with / to cope with / to handle
• 対処した = dealt with / handled / responded to

This is a する-verb noun + する pattern:

• 対処 = handling, dealing with
• する = to do

Past form of する is:

• する → した

So:

• 対処する → 対処した

It can function like the police in English: grammatically one word, but referring to the police as an organization or group.

So 警察 here does not mean one specific police officer. It means the police in general, or the police as an institution.

If you want to refer to an individual officer, you would usually say:

• 警察官 = police officer
• 警官 = policeman / police officer

So:

• 警察が事故に対処した。 = The police dealt with the accident.
• 警察官が事故に対処した。 = A police officer dealt with the accident.

Yes, you could say:

• 警察は事故に対処した。

This is also natural, but the nuance changes.

Very roughly:

• が emphasizes who did it
• は marks 警察 as the topic

So:

• 警察が事故に対処した。 can sound like It was the police who dealt with the accident
• 警察は事故に対処した。 can sound more like As for the police, they dealt with the accident

In many simple contexts, both are possible, but が is often preferred when simply stating the subject of an event.

It is understandable and acceptable, especially in a formal or official-sounding context.

対処する has a somewhat formal, matter-of-fact tone, like deal with or address.

Other possible verbs include:

• 事故に対応した = responded to the accident
• 事故を処理した = processed/handled the accident
• 事故の処理をした = handled the accident case

Nuance differences:

• 対処する = deal with the situation/problem
• 対応する = respond to it
• 処理する = process/handle it, sometimes more bureaucratic or procedural

So your sentence is fine, especially if the style is formal.

Japanese does not have articles like the and a/an.

So:

• 警察 could mean the police
• 事故 could mean the accident or an accident

The exact interpretation comes from context.

That means Japanese often leaves this kind of information unstated when English requires it.

No. Normally, Japanese is written without spaces.

So the usual writing would be:

警察が事故に対処した。

Spaces are often added in beginner materials to make the structure easier to read, but they are not standard in normal Japanese writing.

The structure is:

• 警察 = subject
• が = subject marker
• 事故 = accident
• に = particle required by 対処する
• 対処した = dealt with

So the pattern is:

[subject] が [thing dealt with] に 対処した

or in dictionary form:

[subject] が [thing dealt with] に 対処する

This is a good example of how Japanese often puts the verb at the end, with particles showing each word’s role.