Siinä tapauksessa, että lausunto lisätään jälkikäteen, lähettäkää se selvityksen mukana.

Siinä tapauksessa, että is a formal way to say if / in the event that.

So:

• Siinä tapauksessa, että lausunto lisätään... = If the statement is added...
• A simpler version would be Jos lausunto lisätään...

Using siinä tapauksessa, että sounds more official, administrative, or careful than plain jos.

Because the phrase is literally in that case.

• se → siinä = in it / in that
• tapaus → tapauksessa = in the case

The ending -ssa/-ssä is the inessive case, which often means in.

So the expression is built very literally:

• siinä tapauksessa = in that case

As a whole, though, it functions like a fixed expression meaning if / in the event that.

In this structure, että introduces the clause that explains what the case is.

So:

• siinä tapauksessa = in that case
• että lausunto lisätään jälkikäteen = that the statement is added afterward

Together:

• siinä tapauksessa, että... = in the event that...

In English, that is often optional, but in Finnish this pattern normally uses että.

This is a very common learner question, because Finnish object marking changes depending on the construction.

Here, lausunto is the total object in a passive clause:

• lausunto lisätään = the statement is added / a statement is added

In Finnish, with a passive finite verb, a singular total object is typically in the nominative, so lausunto is correct.

Compare:

• Lisäätte lausunnon. = active clause, total object singular → lausunnon
• Lausunto lisätään. = passive clause, total object singular → lausunto
• Lausuntoa ei lisätä. = negative clause → lausuntoa (partitive)

So the form is not random; it follows Finnish object-case rules.

Lisätään is the Finnish passive form of lisätä.

It means something like:

• is added
• gets added
• someone adds it without saying who

Finnish uses the passive a lot when the doer is unknown, unimportant, or left unspecified.

Here the point is not who adds the statement, but the condition:

• if the statement is added afterward

So the passive is very natural.

Jälkikäteen is an adverb meaning:

• afterward
• later
• after the fact
• sometimes retroactively, depending on context

In this sentence, it means that the statement is not included at first, but is added later.

It is best learned as a fixed word:

• lisätä jälkikäteen = to add afterward / later on

Example:

• Asiakirja toimitettiin jälkikäteen. = The document was submitted afterward.

Lähettäkää is the imperative of lähettää in the second person plural.

It means:

• send
• please send

This form is used when speaking to:

1. more than one person, or
2. one person politely/formally

So in official instructions, lähettäkää is very common.

Compare:

• lähetä = send (to one person, informal)
• lähettäkää = send (to several people, or polite/formal)

Se refers back to lausunto.

So:

• lausunto = the statement
• lähettäkää se = send it

This is completely normal Finnish. Once a thing has been mentioned, se is often used to refer to it again.

English learners sometimes expect something more like it as a special pronoun, but Finnish simply uses se here.

Because mukana is a postposition meaning with / along with, and it requires the word before it to be in the genitive.

So:

• selvitys = report / explanation / account
• selvityksen = genitive form
• selvityksen mukana = with the report / together with the report

This is a standard pattern:

• hakemuksen mukana = with the application
• kirjeen mukana = with the letter

Note that mukana comes after the noun, so it is a postposition, not a preposition.

Finnish normally uses commas to separate a subordinate clause from the main clause.

Here the subordinate clause is:

• että lausunto lisätään jälkikäteen

And the main clause is:

• lähettäkää se selvityksen mukana

So the commas mark the boundaries clearly:

• one before the subordinate clause
• one after it, when the main clause continues

This is very typical Finnish punctuation.

No, not completely, but the current order is natural and formal.

The sentence begins with the condition:

• Siinä tapauksessa, että...

That puts the condition first, which is common in official instructions.

You could rearrange it, but it would sound less natural or less elegant in this context. For example:

• Lähettäkää se selvityksen mukana siinä tapauksessa, että lausunto lisätään jälkikäteen.

This is grammatical, but the original version flows better in formal written Finnish.

Yes. The sentence sounds quite formal and administrative.

A simpler version would be:

• Jos lausunto lisätään jälkikäteen, lähettäkää se selvityksen mukana.

That means the same thing, but it is more straightforward.

So the original is the kind of Finnish you often see in:

• official instructions
• forms
• administrative letters
• public-sector writing