Olen päätöstä vastaan.

Olen is the 1st person singular form of the verb olla (to be).

So:

• olen = I am

That is why the whole sentence starts with Olen ... = I am ...

Because vastaan in this meaning is a postposition that normally requires the partitive case.

So:

• päätös = decision
• päätöstä = decision (in the partitive singular)

In this sentence:

• päätöstä vastaan = against the decision

This is something you mostly just learn as a pattern:

• olla jotakin vastaan = to be against something

Examples:

• Olen tätä ideaa vastaan. = I am against this idea.
• Hän on muutosta vastaan. = He/She is against the change.

Here vastaan means against.

More specifically, it is functioning as a postposition, not as a verb.

So the structure is:

• [noun in partitive] + vastaan
• literally: decision against
• naturally in English: against the decision

A very common Finnish pattern is:

• olla ... vastaan = to be against ...

Because in this use, vastaan is a postposition, and postpositions come after their complement.

English usually uses prepositions:

• against the decision

Finnish here uses a postposition:

• päätöstä vastaan
• literally: the decision against

This is completely normal in Finnish.

Yes. Vastaan has more than one use in Finnish.

Common meanings include:

• against
• to/towards someone in expressions like tulla vastaan = to come toward / to run into
• part of the verb vastata (to answer, to correspond), though that is a different word form

Also, ottaa vastaan means to receive.

So a learner may already have seen vastaan in other expressions. In Olen päätöstä vastaan, though, the meaning is clearly against.

It describes an opinion or stance, not an action.

• Olen päätöstä vastaan. = I am against the decision.

It tells us what the speaker’s position is.

If you want a more active verb, Finnish can also say:

• Vastustan päätöstä. = I oppose the decision.

The difference is roughly:

• Olen päätöstä vastaan = I am against it.
• Vastustan päätöstä = I oppose it.

Both are natural, but the second feels a bit more direct and action-like.

Yes, you can.

Finnish often drops subject pronouns when they are not needed, because the verb form already shows the person:

• olen already tells us the subject is I

So:

• Olen päätöstä vastaan. = neutral, normal
• Minä olen päätöstä vastaan. = more emphatic, contrastive, or explicit

You might use minä if you want to stress I, for example:

• Minä olen päätöstä vastaan, mutta hän kannattaa sitä.
• I am against the decision, but he/she supports it.

Finnish does not usually have articles like a and the.

So päätös can mean:

• a decision
• the decision

The exact meaning comes from context.

In this sentence, if the meaning given to the learner is I am against the decision, that definite meaning comes from the situation, not from a separate word like the.

The dictionary form is päätös.

This is an -s noun, and these nouns often change form when inflected.

Important forms are:

• päätös = decision
• päätöksen = of the decision / the decision (genitive or accusative depending on context)
• päätöstä = decision (partitive)

So when learning vocabulary, you should remember that päätös does not keep exactly the same shape in all cases.

The normal order is:

• Olen päätöstä vastaan.

This is the most straightforward and neutral version.

Because Finnish word order is fairly flexible, other orders are possible for emphasis, but they are less neutral:

• Päätöstä vastaan olen.
• Päätöstä vastaan minä olen.

A learner should stick with:

• Olen päätöstä vastaan.

That is the safest and most natural basic sentence.

You change the form of olla (to be).

Examples:

• Olen päätöstä vastaan. = I am against the decision.
• Olet päätöstä vastaan. = You are against the decision.
• On päätöstä vastaan. = He/She is against the decision.
• Olemme päätöstä vastaan. = We are against the decision.
• Olette päätöstä vastaan. = You are against the decision.
• Ovat päätöstä vastaan. = They are against the decision.

The part päätöstä vastaan stays the same here; only olen/olet/on... changes.