Perustelu puuttuu päätöksestä.

The subject is perustelu.

In Finnish, with the verb puuttua (to be missing / to be absent), the thing that is missing is usually the grammatical subject.

So in:

Perustelu puuttuu päätöksestä.

• perustelu = the missing thing → subject
• puuttuu = is missing
• päätöksestä = from the decision

This is different from English, where learners may first expect something like The decision lacks a justification. Finnish expresses it more like A justification is missing from the decision.

Because puuttua often takes the place or whole from which something is missing in the elative case (-sta / -stä), which often means out of / from.

So:

• päätös = decision
• päätöksestä = from the decision

In this sentence, the idea is:

A justification is missing from the decision.

That is why Finnish uses päätöksestä, not the basic form päätös.

This is a very natural question, because -ssa / -ssä often means in and English might suggest in the decision.

But with puuttua, Finnish usually uses elative (-sta / -stä) rather than inessive (-ssa / -ssä) to mark the source/whole that lacks something.

So Finnish prefers:

• Perustelu puuttuu päätöksestä.

rather than using päätöksessä.

A helpful way to remember it:

• puuttua jostakin = to be missing from something

Puuttua is an intransitive verb, and its structure is important to learn.

A very common pattern is:

• X puuttuu Y:stä
  = X is missing from Y

In your sentence:

• perustelu = X
• päätöksestä = Y:stä

So:

• Perustelu puuttuu päätöksestä.
• literally: Justification is missing from the decision.

This is different from English verbs like lack, where the sentence is often structured the other way around:

• The decision lacks a justification.

Finnish could express that idea differently, but with puuttua, the missing thing becomes the subject.

Because here perustelu is the subject of the sentence, and singular subjects are normally in the nominative.

So:

• perustelu = nominative singular
• puuttuu = 3rd person singular, matching the singular subject

English speakers often expect the missing thing to behave more like an object, but with puuttua, it is not an object. It is the subject.

Compare:

• Perustelu puuttuu. = A justification is missing.

That is why the nominative makes sense here.

Because the subject perustelu is singular.

• perustelu = one justification / one reason
• puuttuu = singular verb form

If the subject were plural, the verb would also be plural:

• Perustelut puuttuvat päätöksestä.
• The justifications are missing from the decision.

So the verb agrees with the subject in number.

Yes. Finnish word order is quite flexible.

You can also say:

• Päätöksestä puuttuu perustelu.

This means essentially the same thing, but the emphasis changes slightly.

A rough comparison:

• Perustelu puuttuu päätöksestä.
  Focuses a bit more on the justification.

• Päätöksestä puuttuu perustelu.
  Focuses a bit more on the decision as the place something is missing from.

Both are natural. Finnish often moves words around for information focus rather than changing the core meaning.

This is a good form to notice because the stem changes.

Dictionary form:

• päätös

Elative singular:

• päätöksestä

What happens:

• päätös changes to the stem päätökse-
• then the elative ending -stä is added

So:

• päätös
• päätökse-
• päätöksestä

This kind of change is normal for many Finnish nouns. It is something you gradually get used to rather than something to panic about.

It can depend on context.

Perustelu can mean:

• justification
• reason
• grounds
• explanation

In legal or administrative language, justification or grounds/reasons is often the best fit.

So in a formal context, this sentence may mean something like:

• The decision lacks justification.
• The grounds are missing from the decision.
• No reasoning is given in the decision.

Even if the exact English wording changes, the Finnish grammar stays the same.

Finnish normally does not use articles.

So perustelu can mean:

• a justification
• the justification

and päätöksestä can mean:

• from a decision
• from the decision

The exact meaning depends on context.

In real usage, this sentence will often be understood as:

• The justification is missing from the decision, or
• A justification is missing from the decision

depending on the situation.

Yes. That is a very natural English rendering.

Even though the Finnish structure is literally closer to:

• A justification is missing from the decision

the meaning often overlaps with:

• The decision lacks a justification

This is a useful reminder not to translate Finnish word-for-word. The important thing is to understand the pattern:

• puuttua jostakin = to be missing from something

Once you know that pattern, the sentence becomes much easier to read.