Valokuva todentaa rekisterinumeron.

Because it is the object of the verb, and in this sentence it is a total object: the photograph verifies the whole registration number as a complete result.

In Finnish, a singular total object in a normal affirmative sentence often has the same form as the genitive singular, which ends in -n.

So:

• rekisterinumero = base form / dictionary form
• rekisterinumeron = total object form here

A learner will often see this explained in two slightly different ways:

• some grammar books call it genitive object
• others call it accusative, but note that for most nouns it looks the same as the genitive

So in practice, for this sentence, the important thing is: -n marks the object as complete/whole.

Rekisterinumeroa would be the partitive form, and that would suggest a different meaning or viewpoint.

The partitive object is often used when:

• the action is incomplete
• the action is ongoing
• only part of something is affected
• the sentence is negative

Here, todentaa presents the result as complete: the photograph verifies the registration number. So Finnish uses the total object instead:

• Valokuva todentaa rekisterinumeron.

Compare:

• Valokuva ei todenna rekisterinumeroa. = The photograph does not verify the registration number.
  Negative sentence -> partitive object.

Valokuva is in the nominative singular, which is the basic form of the noun.

In this sentence, it is the subject:

• valokuva = the photograph / a photograph

So the structure is:

• Valokuva = subject
• todentaa = verb
• rekisterinumeron = object

Because with some Finnish verbs, the 3rd person singular present tense looks exactly like the 1st infinitive (the dictionary form).

So:

• dictionary form: todentaa
• hän/se todentaa = he/she/it verifies

This is very common with verbs of this type, especially verbs ending in -aa or -ää.

For example:

• antaa = to give
• hän antaa = he/she gives

With todentaa, the present tense forms are:

• minä todennan = I verify
• sinä todennat = you verify
• hän/se todentaa = he/she/it verifies
• me todennamme = we verify
• te todennatte = you all verify
• he todentavat = they verify

So in your sentence, todentaa is the finite verb, not just the infinitive.

Right: Finnish has no articles like English a/an and the.

So valokuva can mean:

• a photograph
• the photograph

And rekisterinumeron can mean:

• a registration number
• the registration number

The exact interpretation depends on context.

That means Valokuva todentaa rekisterinumeron could be understood in English as either:

• A photograph verifies the registration number
• The photograph verifies the registration number

depending on the situation.

Yes, it is one compound word: rekisterinumero.

It is made from:

• rekisteri = register, registry
• numero = number

So literally it is something like register number or registration number.

Finnish uses compound nouns very freely, much more than English in some cases. The important thing to notice is that the grammatical ending goes on the last part of the compound:

• rekisterinumero = registration number
• rekisterinumeron = registration number + object ending

No, Finnish word order is fairly flexible because case endings show what each noun is doing.

This sentence uses the most neutral order:

• Valokuva todentaa rekisterinumeron.
• subject + verb + object

But other orders are possible for emphasis or style, for example:

• Rekisterinumeron todentaa valokuva.

That sounds more marked, as if emphasizing it is the photograph that verifies the registration number.

So the given order is the most straightforward, neutral way to say it.

Yes. Valokuva specifically means a photograph or photo.

• valo = light
• kuva = picture, image

So valokuva is literally a light-picture, which makes sense for a photograph.

By contrast, kuva is more general and can mean:

• picture
• image
• illustration
• figure

So:

• valokuva = specifically a photo
• kuva = any kind of picture/image

Todentaa is a fairly formal verb meaning something like:

• verify
• authenticate
• confirm as true

It is the kind of word you might see in:

• official language
• technical documentation
• administrative instructions
• legal or bureaucratic contexts

In everyday speech, people might sometimes prefer other verbs depending on context, such as:

• varmistaa = make sure, ensure, verify
• todistaa = prove
• näyttää toteen = show to be true

So todentaa is perfectly good Finnish, but it has a somewhat formal or official feel.

Both the subject and the verb are singular.

• valokuva = singular
• todentaa = 3rd person singular
• rekisterinumeron = singular object

If the subject were plural, you would see different forms:

• Valokuvat todentavat rekisterinumeron.
• The photographs verify the registration number.

So the original sentence is clearly singular: one photograph verifies one registration number.