Eu sou contra isto.

Because sou comes from ser, not estar.

In Eu sou contra isto, Portuguese uses ser to express a more general position, belief, or stance:

• sou contra = I am against
• sou a favor = I am in favor

Using estar here would usually sound unnatural if you mean a general opinion.
So:

• Eu sou contra isto. = a general stance/opinion
• Estou contra isto. = not the normal way to say it in Brazilian Portuguese

Yes. Very often, Portuguese drops the subject pronoun when it is clear from the verb.

So both are correct:

• Eu sou contra isto.
• Sou contra isto.

Because sou already tells you the subject is I, eu is often optional.

You might keep eu for:

• emphasis
• contrast
• clarity

Example:

• Eu sou contra isto, mas ela é a favor.

Contra means against.

It is a preposition, and in this sentence it introduces the thing you oppose:

• contra isto = against this

Other examples:

• sou contra a ideia = I’m against the idea
• sou contra o projeto = I’m against the project

A very common pair is:

• ser contra = to be against
• ser a favor de = to be in favor of

This is a very common question.

Traditionally, Portuguese has a three-way distinction:

• isto = this near the speaker
• isso = that near the listener or something just mentioned
• aquilo = that over there / that yonder

But in modern Brazilian Portuguese, especially in speech, isso is often much more common than isto.

So although isto is correct, many Brazilians would more naturally say:

• Sou contra isso.

In everyday Brazilian speech:

• isto can sound more formal, written, or less common
• isso is often the default choice

Neither. Isto is a neuter demonstrative pronoun.

That means it refers to:

• an idea
• a situation
• something unspecified
• something not named with a noun

So isto does not change for gender or number.

Compare:

• isto = this thing / this matter / this
• isso = that thing / that matter / that

But if you refer to a specific noun, you use forms that agree with the noun:

• este livro = this book
• esta ideia = this idea
• esses livros = those books
• essas ideias = those ideas

Yes, absolutely — and in Brazilian Portuguese it is often more natural.

So these are both correct:

• Eu sou contra isto.
• Eu sou contra isso.

In many everyday Brazilian contexts, isso is the version you will hear more often.

If you are speaking naturally in Brazil, sou contra isso is usually a very safe choice.

Not here.

With isto, you do not use an article:

• contra isto ✔

But with many nouns, you often will use an article:

• contra o plano = against the plan
• contra a decisão = against the decision
• contra os impostos = against the taxes

So the structure depends on what comes after contra.

The normal word order is:

• Eu sou contra isto.

or, more naturally in many cases:

• Sou contra isto.

You can change the order for emphasis, but that is less neutral:

• Contra isto eu sou. = possible, but unusual and emphatic

For standard, neutral speech, stick with:

• Eu sou contra isto.
• Sou contra isto.

A simple approximate pronunciation is:

• eh-oo soh-o KOHN-trah EES-too

A few notes:

• Eu sounds roughly like eh-oo, but more smoothly in one syllable-like glide
• sou rhymes roughly with English so
• contra has stress on con
• isto has stress on is

In natural Brazilian speech, it may sound more connected and reduced than this spelling suggests.

It is correct, but isto makes it sound a bit more formal or written than the most everyday spoken Brazilian Portuguese.

A very natural everyday version would be:

• Sou contra isso.
• Eu sou contra isso.

So:

• Eu sou contra isto. = correct, slightly more formal/bookish
• Eu sou contra isso. = very natural in Brazil

Yes. You can add words like:

• totalmente = totally
• completamente = completely
• mesmo = really
• de jeito nenhum = no way / absolutely not, in some contexts

Examples:

• Sou totalmente contra isso.
• Eu sou completamente contra isso.

These make your opposition stronger.

The usual opposite is ser a favor de.

Examples:

• Sou contra isso. = I’m against that.
• Sou a favor disso. = I’m in favor of that.

Notice the pattern:

• contra
  • noun/pronoun
• a favor de
  • noun/pronoun

So you say:

• contra isso
• a favor disso

Yes. Because isto is neuter and general, it can refer to many kinds of things:

• an idea
• a plan
• a law
• a behavior
• a situation
• something someone just said or showed

So Eu sou contra isto can mean:

• I oppose this idea
• I’m against this plan
• I disagree with this

The exact meaning depends on context.