Täyttäkää vahinkoilmoitus niin, että kaikki lisätiedot ovat mukana.

Täyttäkää is the imperative form of täyttää, which means to fill in, to complete, or sometimes to fill.

Here it means fill in / complete a form.

So:

• täyttää = to fill in
• täyttäkää = fill in! / please fill in!

This is a command or instruction.

Täytä is the singular imperative: you say it to one person informally.

• Täytä lomake. = Fill in the form. (to one person)

Täyttäkää is the plural imperative: you say it to more than one person.

• Täyttäkää lomake. = Fill in the form. (to several people)

It can also be used as a polite form when addressing one person, a bit like a formal please fill in.

So in this sentence, täyttäkää suggests either:

• the instruction is for several people, or
• it is a polite/formal instruction to one person

This is a very common learner question, because Finnish object forms are tricky.

In Täyttäkää vahinkoilmoitus, vahinkoilmoitus is the object of the verb täyttää.

Here it is in the nominative singular form. That happens because in Finnish, a total object in an affirmative imperative is often in the nominative.

So:

• Täyttäkää vahinkoilmoitus. = Fill in the damage report.
  
  • total object
  • affirmative imperative
  • object appears in nominative singular

Compare:

• Täytätte vahinkoilmoituksen. = You fill in the damage report.
  
  • here the total object is usually vahinkoilmoituksen

So the form is not random: it follows a real object-case rule.

It is a compound noun:

• vahinko = damage, accident, loss
• ilmoitus = notice, report, declaration

So vahinkoilmoitus literally means something like damage report or claim form, depending on context.

In insurance contexts, it is often the form used to report damage or a loss.

Here niin, että means something like:

• so that
• in such a way that

The sentence structure is:

• Täyttäkää vahinkoilmoitus = Fill in the damage report
• niin, että kaikki lisätiedot ovat mukana = so that all additional details are included

So the whole idea is:

Fill in the damage report so that all additional details are included.

This structure is very common in Finnish when explaining how something should be done or with what result.

In Finnish, a comma is normally used before a subordinate clause introduced by words like että.

So in:

• Täyttäkää vahinkoilmoitus niin, että kaikki lisätiedot ovat mukana.

the part starting with että is a subordinate clause:

• että kaikki lisätiedot ovat mukana

That is why the comma appears there.

English punctuation may feel a bit looser in similar sentences, but in Finnish this comma is standard.

Lisätiedot is the plural of lisätieto.

• lisä = extra, additional
• tieto = piece of information, information, detail

So:

• lisätieto = additional information / an extra detail
• lisätiedot = additional details / additional information

Finnish often uses the plural where English might use either details or uncountable information.

So kaikki lisätiedot means:

• all additional details
• or more naturally in English, all additional information

Because the subject is plural.

The subject here is kaikki lisätiedot = all additional details. Since lisätiedot is plural, the verb must also be plural:

• ovat = are

So:

• kaikki lisätiedot ovat mukana = all additional details are included

Compare:

• lisätieto on mukana = the additional detail is included
• lisätiedot ovat mukana = the additional details are included

Mukana literally means with, along, or included, depending on context.

In this sentence, ovat mukana means something like:

• are included
• are present
• are provided as part of it

So kaikki lisätiedot ovat mukana means:

• all additional details are included

This is a very natural Finnish way to express inclusion.

You will also see mukana in other meanings, for example:

• Olen mukana. = I’m coming along / I’m involved.
• Liitteet ovat mukana. = The attachments are included.

Kaikki itself can be a bit tricky, because it can refer to everything or all depending on context.

Here it modifies the plural noun lisätiedot, so it means all:

• kaikki lisätiedot = all additional details

Even though kaikki does not show a plural ending here, the noun after it does, and the verb agrees with that plural noun:

• kaikki lisätiedot ovat mukana

So the whole subject is grammatically plural.

Yes, but the meaning or style would change slightly.

• Täyttäkää vahinkoilmoitus niin, että kaikki lisätiedot ovat mukana.
  = Fill in the damage report so that all additional details are included.

The niin helps create the pattern niin, että..., which strongly means in such a way that... / so that...

Without niin, a sentence with että alone would not sound quite the same here. The pair niin ... että is the natural structure for this kind of instruction.

The sentence has two parts:

1. Täyttäkää vahinkoilmoitus

   • verb first: Täyttäkää
   • object: vahinkoilmoitus

2. niin, että kaikki lisätiedot ovat mukana

   • connector: niin, että
   • subject: kaikki lisätiedot
   • verb: ovat
   • complement: mukana

So the core structure is:

[Command] + [so that-clause]

This is very normal in instructions and formal written Finnish.

Yes, it sounds fairly formal or official.

Reasons:

• täyttäkää is an instruction form often used in notices, forms, and customer service
• vahinkoilmoitus is an administrative/insurance word
• the whole sentence sounds like something you might read on a form or in official guidance

A more everyday spoken version might be simpler, but this sentence is perfectly natural in formal written Finnish.