Είμαι εναντίον αυτής της θεραπείας.

εναντίον means against.

In this sentence, είμαι εναντίον... means I am against... in the sense of being opposed to something.

A very common pattern is:

είμαι εναντίον + genitive noun/pronoun
= I am against + something/someone

So:

• Είμαι εναντίον αυτής της θεραπείας. = I am against this treatment.

It is usually used for opinions, opposition, or disagreement, not physical movement.

Because εναντίον takes the genitive case.

The basic dictionary form is:

• η θεραπεία = the treatment (nominative)

But after εναντίον, the noun phrase must go into the genitive:

• της θεραπείας = of the treatment
• αυτής της θεραπείας = of this treatment

So the Greek structure is literally closer to:

• I am against of this treatment

Even though in natural English we simply say against this treatment.

Because Greek often uses both:

1. a demonstrative = this
2. the article = the
3. the noun

So:

• αυτής = this (genitive feminine singular)
• της = the (genitive feminine singular)
• θεραπείας = treatment (genitive feminine singular)

Together:

• αυτής της θεραπείας = this treatment

This is very normal Greek. The article usually stays there even when you use this or that.

Compare:

• αυτή η θεραπεία = this treatment (nominative)
• αυτής της θεραπείας = of this treatment / this treatment after a word requiring genitive

Because it has to agree with θεραπείας.

θεραπεία is:

• feminine
• singular
• here in the genitive

So the demonstrative this must also be:

• feminine
• singular
• genitive

That gives:

• αυτής

Useful comparison:

• αυτή η θεραπεία = this treatment (nominative)
• αυτής της θεραπείας = this treatment / of this treatment (genitive)

So αυτής is not a different word meaning something else; it is just the correct grammatical form of this for this context.

Because εναντίον requires the genitive.

The noun θεραπεία changes form:

• η θεραπεία = nominative singular
• της θεραπείας = genitive singular

After εναντίον, you say:

• εναντίον της θεραπείας
• εναντίον αυτής της θεραπείας

This is something you often just have to learn together with the word:

• εναντίον + genitive

Many learners find it helpful to memorize it as a whole pattern rather than trying to translate word by word.

είμαι means I am.

It is the 1st person singular of the verb είμαι = to be.

So:

• είμαι = I am
• είσαι = you are
• είναι = he/she/it is

In this sentence:

• Είμαι εναντίον αυτής της θεραπείας.
• I am against this treatment.

You need είμαι here because Greek, like English, uses the verb to be in this kind of statement.

A simple learner-friendly pronunciation is:

EE-meh en-an-DEE-on af-TEES teez the-ra-PEE-as

A few notes:

• Είμαι sounds like EE-meh
• εναντίον has the stress on -τί-
• αυτής is roughly af-TEES
• της is teez
• θεραπείας has stress on -εί-, so roughly the-ra-PEE-as

Also, θ in Greek is like the th in think, not the th in this.

Yes, Greek word order is more flexible than English, although some orders sound more neutral than others.

The most neutral version is:

• Είμαι εναντίον αυτής της θεραπείας.

But you may also hear:

• Αυτής της θεραπείας είμαι εναντίον.

This sounds more marked or emphatic, as if stressing this treatment in particular.

For learners, the safest and most natural order to use is:

• Είμαι εναντίον αυτής της θεραπείας.

Yes, κατά can also mean against, and in many contexts it is very similar to εναντίον.

So:

• Είμαι κατά αυτής της θεραπείας.
• Είμαι εναντίον αυτής της θεραπείας.

Both can mean I am against this treatment.

However, learners will often encounter:

• εναντίον + genitive
• κατά + genitive

Both are common in more formal or argumentative contexts.
In everyday speech, some people may also prefer other phrasing depending on tone and situation.

So your original sentence is completely natural and correct.

It is mostly neutral.

• θεραπεία is a normal word for treatment/therapy
• εναντίον is a standard word for against
• the whole sentence sounds natural in conversation, writing, or discussion

Depending on context, it can sound a little serious because it expresses a clear opinion or objection, but it is not especially formal or bookish.

A common opposite is:

• Είμαι υπέρ αυτής της θεραπείας.

Here:

• υπέρ = in favor of
• and it also takes the genitive

So compare:

• Είμαι εναντίον αυτής της θεραπείας. = I am against this treatment.
• Είμαι υπέρ αυτής της θεραπείας. = I am in favor of this treatment.

This is a very useful pair to learn together:

• εναντίον = against
• υπέρ = in favor of

You keep εναντίον αυτής της θεραπείας the same, and change the form of είμαι.

Examples:

• Είσαι εναντίον αυτής της θεραπείας. = You are against this treatment.
• Είναι εναντίον αυτής της θεραπείας. = He/She/It is against this treatment.
• Είμαστε εναντίον αυτής της θεραπείας. = We are against this treatment.
• Είστε εναντίον αυτής της θεραπείας. = You are against this treatment. / You all are against this treatment.
• Είναι εναντίον αυτής της θεραπείας. = They are against this treatment.

So the main thing that changes is the form of to be.

You put δεν before είμαι:

• Δεν είμαι εναντίον αυτής της θεραπείας.

This means:

• I am not against this treatment.

Important: not against does not always mean in favor of. It may simply mean neutral or undecided.

So:

• Είμαι εναντίον... = I am against...
• Δεν είμαι εναντίον... = I am not against...

Not always. θεραπεία can mean:

• treatment
• therapy

The best translation depends on context.

For example:

• in a medical context, θεραπεία may be treatment
• in a psychological or rehabilitation context, it may be therapy

So this sentence could also sometimes be understood as:

• I am against this therapy.

If the meaning shown to the learner is treatment, that is perfectly natural.