kono syasin ga tegakari ni natta.

The structure is:

• この写真 = this photo
• が = marks the thing being focused on
• 手がかり = clue, lead
• に = marks the result/state after a change
• なった = became

So the pattern is:

A が B に なる = A becomes B

In this sentence:

この写真が手がかりになった
= This photo became a clue
or more naturally, This photo turned out to be a clue / This photo served as a clue.

が highlights this photo as the thing that became the clue.

So the sentence has a sense like:

• It was this photo that became the clue
• This photo is what served as the clue

If you used は instead:

この写真は手がかりになった

that would sound more like you are introducing this photo as the topic and then commenting on it: As for this photo, it became a clue.

Both can be possible in context, but が is very natural when identifying or focusing on the specific thing that played that role.

Here, に marks the resulting state of a change.

In B に なる, に shows what something changes into.

Examples:

• 先生になる = become a teacher
• 春になる = become spring / turn into spring
• 元気になる = become well/healthy

So in:

手がかりになった

the meaning is became a clue.

This is different from location に such as:

• 学校に行く = go to school
• 机にある = be on the desk

Same particle, different function.

Because the sentence is describing a change or result, not simply saying what something was.

• 手がかりだった = was a clue
• 手がかりになった = became a clue

So なった emphasizes that the photo came to function as a clue at some point in the situation.

This often sounds more natural in Japanese when something newly proves useful or reveals its importance.

手がかり means a clue, lead, or something that helps you figure something out.

It is often used in:

• investigations
• solving mysteries
• finding a person
• understanding a situation

So 手がかりになった often means more than just was a clue. It can suggest:

• it helped move things forward
• it gave people a lead
• it was the key hint that helped them discover something

A natural English translation may vary depending on context:

• This photo became a clue
• This photo provided a clue
• This photo led them in the right direction

Yes, possibly.

Japanese often leaves out information that is understood from context. This sentence does not explicitly say for whom the photo became a clue.

Depending on the larger context, it could mean:

• it became a clue for us
• it became a clue for the police
• it became a clue in the investigation
• it became a clue to solving the mystery

Japanese does this very often. A sentence can be complete even when English would normally add extra words.

この means this and must directly modify a noun.

So:

• この写真 = this photo

You cannot use この by itself.

Compare:

• この写真 = this photo
• これ = this

So if you want to say this photo, この写真 is the correct form.

Yes. Japanese nouns do not use articles like a, an, or the.

So 手がかり by itself can mean:

• a clue
• the clue
• clues in some contexts

The exact English wording depends on context.

That is why 手がかりになった could be translated as:

• became a clue
• became the clue
• became an important lead

Japanese leaves that kind of distinction unstated unless something else makes it clear.

The difference is:

• 手がかりだった = it was a clue
• 手がかりになった = it became a clue / turned out to be a clue

だった is more static. It describes the thing as a clue.

になった is more dynamic. It suggests a development:

• at first it was just a photo
• later it turned out to help
• as events unfolded, it became important

So になった often feels more natural when the sentence is about discovery or progress.

To some extent, yes, but this order is very standard and natural.

この写真が手がかりになった is straightforward:

• the thing: この写真
• marker: が
• result: 手がかりに
• verb: なった

Japanese verbs usually come at the end, and the noun + particle combinations can sometimes move around, but this sentence is already in the most neutral, easy-to-understand order.

It is read てがかり.

It can also be written as 手掛かり, but in everyday Japanese it is very often written as 手がかり in mixed kanji and hiragana, or even fully in hiragana.

This is common in Japanese when:

• the full kanji form is less common
• the kana version is easier to read
• the word is more familiar in kana spelling

So 手がかり is completely normal spelling.