Voisitteko perustella tämän päätöksen selkeästi?

Voisitteko is made of three parts:

• voi- = the stem of voida (to be able / can)
• -sitte- = the conditional 2nd person plural ending
• -ko = the yes/no question particle

So voisitteko literally means something like would you be able to...?, but in natural English it is usually just could you...?

In this sentence, it makes the request polite:

• Voisitteko perustella... ? = Could you justify/explain... ?

Because voisitteko sounds more polite and less direct.

Compare:

• Voitko perustella tämän päätöksen? = Can you justify this decision?
• Voisitteko perustella tämän päätöksen? = Could you justify this decision?

The conditional form voisi- often softens requests in Finnish, just like could does in English.

So the speaker is not really talking about ability; they are making a polite request.

Not necessarily.

In Finnish, the 2nd person plural form can be used:

• for several people
• or as a polite/formal way to address one person

So voisitteko can mean either:

• could you all...?
• or polite singular could you...?

This is similar to formal address in some other European languages, though it is less central in modern everyday Finnish than in some languages. Still, it is very natural in formal or respectful contexts.

-Ko/-kö turns a statement into a yes/no question.

For example:

• Voisitte perustella tämän päätöksen. = You could justify this decision.
• Voisitteko perustella tämän päätöksen? = Could you justify this decision?

In Finnish, this question particle usually attaches to the first important word in the clause, very often the finite verb.

Because it follows voida (can / could / be able to).

In Finnish, after a modal verb like voida, the main action is usually in the basic infinitive form:

• voin tehdä = I can do
• voit sanoa = you can say
• voisitteko perustella = could you justify

So perustella stays in the infinitive because voisitteko is the conjugated verb in the sentence.

They are related, but not identical.

• perustella = to justify, to give reasons for, to argue the grounds for something
• selittää = to explain

So perustella is more specifically about giving the reasoning behind something.

For example:

• Selittäkää tämä asia. = Explain this matter.
• Perustelkaa tämä päätös. = Justify this decision / give the reasons for this decision.

In your sentence, perustella fits well because the speaker wants the reasons behind the decision, not just any explanation.

Because this phrase is the object of the verb perustella, and in this sentence it is a complete, specific object.

Finnish often marks a complete object in the singular with a form that looks like the genitive:

• päätös = decision (basic dictionary form)
• päätöksen = object/genitive-looking form

And the demonstrative tämä must match it:

• tämä päätös = this decision as a subject/basic form
• tämän päätöksen = this decision as the object here

So:

• Tämä päätös on tärkeä. = This decision is important.
• Voisitteko perustella tämän päätöksen? = Could you justify this decision?

This is a normal stem change for this noun type.

The dictionary form is:

• päätös = decision

But many case forms use the stem:

• päätökse-

So you get:

• päätöksen
• päätöksessä
• päätöksestä
• päätökseen

This is just part of how this word inflects; it is not something special caused only by this sentence.

A few similar words behave in comparable ways, so it is worth learning päätös together with some case forms, not only the dictionary form.

Selkeästi is an adverb meaning clearly.

It comes from the adjective:

• selkeä = clear

and the adverb ending:

• -sti

So:

• selkeä = clear
• selkeästi = clearly

This is very common in Finnish:

• nopea = fast
• nopeasti = quickly
• varma = certain
• varmasti = certainly

In the sentence, selkeästi describes how the decision should be justified.

Because that is a very natural place for an adverb of manner in Finnish.

The sentence structure is:

• Voisitteko = could you
• perustella = justify
• tämän päätöksen = this decision
• selkeästi = clearly

So the adverb comes after the object and tells us how the action should be done.

Other word orders are possible, but they may shift emphasis:

• Voisitteko perustella tämän päätöksen selkeästi? = neutral, natural
• Voisitteko selkeästi perustella tämän päätöksen? = puts a bit more emphasis on clearly

Finnish word order is flexible, but the version you have is very standard.

Yes, but the tone changes.

• Voitteko perustella tämän päätöksen selkeästi? = Can you justify this decision clearly?
• Voisitteko perustella tämän päätöksen selkeästi? = Could you justify this decision clearly?

The first sounds more direct and practical. The second sounds softer and more polite.

So both are grammatical, but voisitteko is better if the speaker wants to sound respectful or diplomatic.

In some contexts, yes, but it would not be the most straightforward choice here.

Tämän päätöksen is the normal form when referring to a specific whole decision as the object of a positive request.

Tätä päätöstä is the partitive form, and partitive objects appear in certain situations, for example:

• in negative sentences
• with ongoing/incomplete actions
• with some verbs and meanings

A useful contrast is:

• Voisitteko perustella tämän päätöksen? = Could you justify this decision?
• Ettekö voisi perustella tätä päätöstä? = Couldn't you justify this decision?
  Here the negative verb triggers the partitive.

So in your original sentence, tämän päätöksen is the expected form.