Jestem przeciw tej decyzji.

Because przeciw / przeciwko takes the dative case in Polish.

• base form: ta decyzja = this decision
• accusative: tę decyzję
• dative: tej decyzji

So after przeciw, you need tej decyzji, not tę decyzję.

It is dative singular.

More specifically:

• decyzja is a feminine singular noun
• tej is the feminine singular form of ten / ta / to in the dative
• decyzji is the dative singular form of decyzja

So the whole phrase tej decyzji means to/for this decision in form, even though in English we translate it as against this decision because of the preposition przeciw.

Because Polish words like this change depending on case, gender, and number.

The dictionary form is:

• masculine: ten
• feminine: ta
• neuter: to

But here decyzja is feminine and also in the dative, so ta becomes tej.

So:

• ta decyzja = this decision
• przeciw tej decyzji = against this decision

Almost yes. Both can mean against.

In this sentence, both are possible:

• Jestem przeciw tej decyzji.
• Jestem przeciwko tej decyzji.

The shorter przeciw is very common, especially in expressions like jestem przeciw.
Przeciwko can sound a little fuller or more careful, but the meaning is basically the same here.

Yes, it is natural and common.

It is especially common when expressing an opinion, objection, or vote:

• Jestem przeciw tej decyzji.
• Jestem przeciw temu pomysłowi.

A slightly different but also very natural version is:

• Jestem przeciwny tej decyzji.

That version uses the adjective przeciwny = opposed.
So both are good, but jestem przeciw is a very common everyday pattern.

Because Polish often drops subject pronouns when they are already clear from the verb form.

• jestem already means I am
• so ja is not necessary

You could say:

• Ja jestem przeciw tej decyzji.

But that adds emphasis, something like:

• I am against this decision
• As for me, I’m against this decision

The neutral version is simply:

• Jestem przeciw tej decyzji.

Jestem is the 1st person singular present tense of być = to be.

So:

• jestem = I am

That is why the sentence starts with Jestem... = I am...

Yes. That is a very good alternative.

Compare:

• Jestem przeciw tej decyzji.
• Jestem przeciwny tej decyzji.

Both mean roughly I am against this decision.

The difference is mainly structure:

• przeciw is a preposition/adverb-like word in this expression
• przeciwny is an adjective meaning opposed

Both are natural.
Some learners find przeciwny easier because it matches English opposed to more closely.

Yes, Polish word order is more flexible than English, but not all orders sound equally neutral.

The most neutral order is:

• Jestem przeciw tej decyzji.

You could also move things for emphasis, for example:

• Tej decyzji jestem przeciw.

That sounds more contrastive or emphatic, as if you are stressing this decision.

So yes, word order can change, but the original sentence is the most normal default version.

It can do both, depending on context.

It may mean:

• I am against this decision as an opinion
• I vote against this decision in a meeting or formal discussion

Polish often leaves that distinction to context.

Because Polish dative endings depend on the noun’s gender and declension pattern.

Decyzja is a feminine noun ending in -ja. Its dative singular is:

• decyzji

This is normal for many feminine nouns of this type.

For comparison:

• książka → książce
• kobieta → kobiecie
• decyzja → decyzji

So decyzji is simply the correct dative singular form for this noun.

It is neutral and works in both everyday and fairly formal situations.

You could use it:

• in conversation
• in a meeting
• in a discussion
• in writing, depending on style

If you want something a little more formal or explicit, you might also hear:

• Sprzeciwiam się tej decyzji. = I oppose this decision.
• Jestem przeciwny tej decyzji.

But Jestem przeciw tej decyzji is completely normal and correct.