Valokuva on tärkeä todiste.

Word by word:

• valokuva = photograph / photo
• on = is
• tärkeä = important
• todiste = evidence / proof / a piece of evidence

A very natural English translation is A photograph is important evidence or The photograph is important evidence, depending on context.

Because Finnish does not have articles.

So valokuva can mean:

• a photograph
• the photograph
• sometimes just photograph in a general sense

The context tells you which one is meant.

On is the 3rd person singular present tense of olla, which means to be.

So:

• minä olen = I am
• sinä olet = you are
• hän on = he/she is
• valokuva on = the photograph is

Since valokuva is singular, on is the correct form.

Yes. Valokuva is the subject.

The sentence structure is:

• Valokuva = subject
• on = verb
• tärkeä todiste = what the subject is being identified as

So the sentence is saying that the photograph is an important piece of evidence.

Here, tärkeä todiste is a predicate noun phrase after on.

In this kind of sentence, Finnish often uses the nominative (the basic dictionary form), especially in simple singular statements like this one.

So:

• tärkeä = nominative singular
• todiste = nominative singular

This is why you do not see endings like -n, -a/-ä, and so on here.

Directly, tärkeä describes todiste.

So:

• tärkeä todiste = important evidence / an important piece of evidence

Then the whole phrase tärkeä todiste tells you what valokuva is.

So the logic is:

• the photograph = an important piece of evidence

This is a very good question, because Finnish and English work differently here.

In Finnish, todiste is a normal singular countable noun. It often means:

• proof
• a piece of evidence
• sometimes just evidence in translation

So tärkeä todiste is often most literally an important piece of evidence.

English often prefers uncountable evidence, but Finnish is perfectly happy with singular todiste.

Yes, Finnish word order is more flexible than English word order.

The neutral, basic version is:

• Valokuva on tärkeä todiste.

You can change the order for emphasis, but the meaning or focus may shift. For example:

• Tärkeä todiste on valokuva.

This sounds more marked and emphasizes tärkeä todiste or contrasts valokuva with something else.

For a learner, the original order is the safest and most natural one to use.

Yes.

It is made from:

• valo = light
• kuva = picture / image

So valokuva literally has the idea of a light-picture, which makes sense for photograph.

This is very common in Finnish: many nouns are compounds.

The letter ä is pronounced somewhat like the vowel in English cat for many speakers.

So tärkeä is roughly:

• tär-ke-ä

A few useful points:

• stress is on the first syllable
• the final eä is pronounced in separate syllables, not merged into one
• Finnish pronunciation is usually very regular, so letters are pronounced consistently

So the whole sentence is pronounced with first-syllable stress in each word:

• VA-lo-ku-va on TÄR-ke-ä TO-dis-te

You would say:

• Tämä valokuva on tärkeä todiste.

Here:

• tämä = this
• valokuva = photograph
• on = is
• tärkeä todiste = important evidence / an important piece of evidence