Päätös pitää oikaista.

Because pitää has more than one meaning in Finnish.

In the pattern pitää + infinitive, it usually means must / have to:

• pitää mennä = must go
• pitää oikaista = must correct / must rectify

So in Päätös pitää oikaista, this is the necessity use of pitää, not the everyday verb to hold, to keep, or to like.

Oikaista is the first infinitive, the basic dictionary form of the verb.

After pitää meaning must, Finnish uses this infinitive form:

• pitää tehdä = must do
• pitää mennä = must go
• pitää oikaista = must correct

So nothing special has happened to oikaista here; it is simply the normal infinitive after a modal-like verb.

Because Finnish often expresses this idea differently from English.

A very literal analysis is closer to:

• someone must correct the decision

But the someone is not stated. Since English usually wants a smoother, more natural sentence in that situation, it often uses the passive:

• The decision must be corrected

So the Finnish sentence has an impersonal necessity meaning, even though oikaista itself is not in a passive form here.

This is a very common learner question.

In necessive constructions with verbs like pitää and täytyy, a total object often appears in the nominative, not in the usual genitive/accusative-looking form.

So:

• Minun pitää lukea kirja = I must read the book
• Päätös pitää oikaista = The decision must be corrected

Here päätös is the whole thing that needs correcting, so it is treated as a total object, and in this construction that total object is typically nominative.

Not really in terms of meaning.

The decision is not doing the action. It is the thing being affected by the action of oikaista.

A useful learner-friendly way to think about it is:

• underlying idea: someone must correct the decision

So päätös is better understood as the thing being corrected, not the doer. In many grammatical descriptions, this kind of sentence is treated as having no normal subject.

In this must / have to meaning, pitää normally stays in the 3rd person singular form.

For example:

• minun pitää
• sinun pitää
• meidän pitää

So even when the logical doer changes, pitää itself usually does not.

In Päätös pitää oikaista, there is no expressed doer at all, so pitää remains in its normal necessive form.

Yes. Finnish can leave the agent unstated very naturally.

That is exactly what happens here: the sentence says that correcting the decision is necessary, but it does not say who must do it.

If you want to add the responsible person or institution, you can do so:

• Viranomaisen pitää oikaista päätös. = The authority must correct the decision.
• Meidän pitää oikaista päätös. = We must correct the decision.

So the original sentence is intentionally impersonal.

The basic idea of oikaista is to straighten or to make right.

Depending on context, it can mean things like:

• straighten
• correct
• rectify
• amend

With päätös, especially in an administrative or legal context, it usually means correct or rectify a decision.

So this sentence sounds fairly formal and could easily appear in official language.

Yes. That is very natural and means almost the same thing.

• Päätös pitää oikaista
• Päätös täytyy oikaista

Both mean The decision must be corrected.

Very roughly:

• pitää and täytyy are both common for necessity
• täytyy can sometimes sound a bit more straightforward or explicit
• in many contexts, the difference is small

Yes. A more formal written alternative is:

• Päätös on oikaistava.

This also means The decision must be corrected.

Compared with Päätös pitää oikaista, this version sounds more official, compact, and written.

Because Finnish word order is flexible, and putting päätös first makes it the topic of the sentence.

So the sentence is basically saying:

• As for the decision, it must be corrected

This is natural when the decision is already what you are talking about.

The meaning would stay similar if the wording changed, but the emphasis would shift. Finnish often uses word order to show what is already under discussion and what is new or important information.