Todistusaineisto sisältää valokuvan.

Yes. Finnish very often forms nouns as compounds, written as one word.

todistusaineisto = todistus + aineisto

• todistus relates to proving, evidence, testimony, proof
• aineisto means material, data, collected material

So todistusaineisto is literally something like evidence material or evidentiary material. In Finnish, the last part of the compound is the main noun, so this is a kind of aineisto.

Here todistusaineisto is the subject of the sentence, so it is in the nominative singular, which is the basic dictionary form.

That is why you see:

• todistusaineisto sisältää ...
• not todistusaineiston sisältää ...

In a basic statement, the subject is normally nominative, and the verb agrees with it.

Because the subject todistusaineisto is third-person singular (it), the verb is in the third-person singular present tense:

• sisältää = contains

A useful thing to notice: with many Finnish verbs ending in -taa / -tää, the infinitive and the 3rd person singular present look the same in writing.

So here:

• dictionary form: sisältää = to contain
• sentence form: sisältää = contains

You tell the function from the sentence context.

Because valokuvan is the object of the verb, and in this sentence it is a total object: the sentence refers to one whole photograph.

So:

• valokuva = photo / photograph (basic form)
• valokuvan = a/the whole photograph as the object

In beginner-friendly terms, this -n form is what you often get with a singular complete object in a normal affirmative sentence.

Valokuvaa would be the partitive form, and that usually gives a different meaning or appears in different grammar environments.

Compare:

• Todistusaineisto sisältää valokuvan.
  = The evidence contains a whole photograph / one photograph

• Todistusaineisto ei sisällä valokuvaa.
  = The evidence does not contain a photograph
  (after negation, Finnish usually uses the partitive)

The partitive can also suggest an incomplete amount, an unspecified quantity, or something not viewed as a whole.

So in your sentence, valokuvan is used because the photograph is treated as a complete, countable item.

This is one of those places where Finnish grammar terminology can confuse learners.

Formally, valokuvan looks exactly like the genitive singular. But in this sentence, its function is that of a total object. In many teaching materials, that object form is explained as accusative, even though for most singular nouns it has the same shape as the genitive.

So the practical learner answer is:

• form: valokuvan
• function: singular total object
• shape: the same as the genitive singular

You do not need to worry too much about the label at first; the important thing is understanding why the object gets -n here.

Finnish has no articles, so there is no direct equivalent of a/an/the.

That means valokuvan can mean:

• a photograph
• the photograph

depending on context.

The same is true for todistusaineisto. Depending on the wider situation, it could be understood as:

• the evidence
• the evidentiary material
• just evidence material

English forces you to choose an article; Finnish often leaves that to context.

This is the neutral word order:

• Todistusaineisto sisältää valokuvan.
• subject + verb + object

That is the most natural order for a plain statement.

Finnish word order is more flexible than English, though. You can move words for emphasis, contrast, or topic/focus. For example, putting valokuvan first would give it more emphasis. But the sentence you have is the straightforward, unmarked version.

It would be:

Todistusaineisto ei sisällä valokuvaa.

Two important changes happen:

1. sisältää becomes ei sisällä
   This is how Finnish forms the negative in the present tense.

2. The object changes from valokuvan to valokuvaa
   After negation, Finnish normally uses the partitive object.

This is a very important pattern in Finnish:

• affirmative total object: valokuvan
• negative object: valokuvaa

A rough syllable breakdown is:

to-dis-tus-ai-neis-to si-säl-tää va-lo-ku-van

A few key pronunciation points:

• Stress is on the first syllable of each word:

  • TOdistusaineisto
  • SIsältää
  • VAlokuvan

• ä is a front vowel, somewhat like the vowel in English cat, but purer.

• Double vowels are long:

  • tää in sisältää is held longer than a short vowel.

• Finnish spelling is very regular: letters are usually pronounced consistently.

So if you say the sounds clearly and keep the first-syllable stress, you will already be quite close.