Tarkistin rekisterinumeron ennen tiedoksiantoa.

Tarkistin is the 1st person singular past tense of tarkistaa.

• tarkistaa = to check, inspect, verify
• tarkistin = I checked

So the ending -n shows I, and the past marker is -i-.

Compare:

• tarkistan = I check / I am checking
• tarkistin = I checked

In Finnish, the subject pronoun is often left out when the verb ending already makes the person clear.

So:

• Tarkistin already means I checked
• Minä tarkistin also means I checked, but minä is usually only added for emphasis or contrast

This is very normal in Finnish.

Because it is the object of the verb, and here it is a total object in the singular.

In this sentence, the checking is understood as a completed action affecting the whole registration number, so Finnish uses the genitive-looking object form:

• basic form: rekisterinumero
• object here: rekisterinumeron

Many learners first think of this simply as the object ending -n. More precisely, it is the singular total object, which often looks identical to the genitive singular.

The difference is mainly about how the object is viewed.

• rekisterinumeron = the whole registration number was checked; the action is seen as complete
• rekisterinumeroa = partitive object; this would suggest an incomplete, ongoing, or non-result-focused action

So in your sentence, rekisterinumeron fits because the speaker checked the number as a completed act.

Yes. Finnish forms compounds very freely, and rekisterinumero is a compound noun:

• rekisteri = register
• numero = number

Together:

• rekisterinumero = registration number / register number

A useful rule: in Finnish compounds, usually only the last part inflects.

So:

• rekisterinumero
• rekisterinumeron
• rekisterinumeroa

Because ennen takes the partitive when it is followed by a noun.

So:

• ennen tiedoksiantoa = before the service/notification

Here:

• basic form: tiedoksianto
• after ennen: tiedoksiantoa (partitive singular)

This is something you often just have to learn with the word ennen.

It is a noun. It is also a compound-like formation common in formal or legal Finnish.

Very roughly, it is built from:

• tiedoksi = for information / to someone’s knowledge
• anto = giving

So tiedoksianto is something like giving notice or official notification/service.

Even if the internal structure is interesting, in practice you should learn it as a single noun:

• tiedoksianto
• tiedoksiantoa
• tiedoksiannon, etc.

Yes. That is exactly what ennen tiedoksiantoa tells you.

The structure is:

• Tarkistin = I checked
• rekisterinumeron = the registration number
• ennen tiedoksiantoa = before the notification/service

So the timeline is:

1. first, the checking happened
2. later, the tiedoksianto happened

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

For example, these are all possible depending on emphasis:

• Tarkistin rekisterinumeron ennen tiedoksiantoa.
• Ennen tiedoksiantoa tarkistin rekisterinumeron.

The second version puts more focus on the time expression before the notification/service.

Because Finnish uses case endings, the sentence can still stay clear even when the order changes.

Finnish does not have articles like English a/an and the.

So nouns appear without them:

• rekisterinumero
• tiedoksianto

Whether English uses a or the depends on context and translation, not on a separate Finnish word.

That is why a Finnish sentence can feel very compact to an English speaker.

Yes, it sounds fairly formal, especially because of the word tiedoksianto, which is common in administrative or legal contexts.

The grammar itself is normal Finnish, but the vocabulary is not especially casual. In everyday speech, a different noun might be used depending on the situation.