keisatu ga tegakari wo mituketa.

These are particles showing each word’s role in the sentence.

• が marks the subject: 警察が = the police is the one doing the action.
• を marks the direct object: 手がかりを = a clue / the clue is what was found.

So the structure is:

警察 が 手がかり を 見つけた。
[subject] + [object] + [verb]

This is a very basic and common Japanese sentence pattern.

Both can be possible, but they give slightly different nuances.

• 警察が手がかりを見つけた。
  This sounds like The police found a clue with focus on who found it, or it simply presents new information.

• 警察は手がかりを見つけた。
  This sounds more like As for the police, they found a clue, or it contrasts the police with someone else.

In simple standalone sentences, が is often used when introducing or identifying the subject more directly.

警察 usually means the police as an institution or group, not a single officer.

So in this sentence, it most naturally means:

• the police
• possibly the police department
• or police investigators, depending on context

If you specifically wanted a police officer, you would usually say 警察官.

Japanese nouns usually do not show singular vs. plural the way English does.

So 警察 does not itself tell you whether it is:

• the police as a group
• police officers
• or a police organization

The same is true for 手がかり: it could mean a clue or clues, depending on context. English forces you to choose singular or plural, but Japanese often leaves that open unless it matters.

Japanese does not have articles like a, an, or the.

So 手がかり could mean:

• a clue
• the clue
• clues

and 警察 could mean:

• the police
• police

You figure out the most natural English translation from context. That is why one Japanese sentence can sometimes be translated in more than one correct way.

見つけた is the plain past form of the verb 見つける, which means to find or to discover.

So:

• dictionary form: 見つける
• plain past: 見つけた
• polite past: 見つけました

In this sentence, 見つけた means found.

Japanese is normally a subject-object-verb language, so the verb usually comes last.

English:

• The police found a clue.
• subject + verb + object

Japanese:

• 警察が手がかりを見つけた。
• subject + object + verb

This verb-final structure is one of the most important differences between Japanese and English.

They are different verbs:

• 見た = saw
• 見つけた = found

So:

• 手がかりを見た = saw the clue
• 手がかりを見つけた = found the clue

見つける has the idea of discovering something, often after searching or noticing it.

It is read:

けいさつ が てがかり を みつけた。

Word by word:

• 警察 = けいさつ
• 手がかり = てがかり
• 見つけた = みつけた

Also note that を is written を but pronounced basically like o.

No. 手がかり can be a physical clue, but it can also mean a more abstract lead, hint, or key piece of information.

Depending on context, it might refer to:

• evidence in an investigation
• a hint for solving a problem
• a starting point for understanding something

So it is a flexible word, not just a detective-story word.

The spaces are just for learners. Normal Japanese writing usually does not use spaces between words.

A natural written version would be:

警察が手がかりを見つけた。

Textbooks and beginner materials often add spaces to make it easier to see the word and particle boundaries.

Yes. The polite version would be:

警察が手がかりを見つけました。

The meaning is basically the same, but the tone is more polite or formal.

Compare:

• 見つけた = plain
• 見つけました = polite

The plain form is very common in dictionaries, casual speech, stories, and many example sentences.

Yes, Japanese word order is more flexible than English, as long as the particles stay attached to the right words.

For example:

• 警察が手がかりを見つけた。
• 手がかりを警察が見つけた。

Both are possible. The basic meaning stays the same because:

• が still marks the subject
• を still marks the object

However, changing the order can change the emphasis or what sounds most natural in context. The version you were given is the most straightforward neutral order.