Eles são contra o plano.

A word-by-word breakdown:

• Eles = they (masculine or mixed-gender group)
• são = are (3rd person plural of ser)
• contra = against
• o = the (masculine singular)
• plano = plan

So the structure is very close to English: They are against the plan.

Because eles is they, which is plural.

The verb ser has to agree with the subject:

• eu sou = I am
• você/ele/ela é = you/he/she is
• nós somos = we are
• eles/elas são = they are

So:

• Ele é contra o plano. = He is against the plan.
• Eles são contra o plano. = They are against the plan.

In Portuguese, ser contra is the normal way to say to be against something in the sense of opinion, position, or stance.

So:

• sou contra = I am against
• somos contra = we are against
• eles são contra = they are against

This is a very common pattern in Portuguese.

Usually, no.

Ser contra is the normal expression for having an opinion or position:

• Eles são contra o plano. = They are against the plan.

Using estar contra would sound unnatural in most everyday situations.

A good rule here is:

• for opinions/positions like in favor of / against, use ser
• estar is more for temporary states, locations, conditions, etc.

Contra is a preposition, and it does not change for gender or number.

So it stays the same in all these examples:

• contra o plano = against the plan
• contra a ideia = against the idea
• contra os planos = against the plans
• contra as regras = against the rules

Unlike adjectives, it does not become different forms such as masculine/feminine/plural.

O is the definite article the.

So:

• o plano = the plan

In Portuguese, articles are often used more regularly than in English. In this sentence, o is the natural choice because it refers to a specific plan.

Usually, that would sound incomplete or unnatural.

The normal sentence is:

• Eles são contra o plano.

Without the article, Portuguese often sounds less natural unless there is some very specific context or a more abstract/general meaning. For most learners, it is best to use contra o plano here.

Because plano is a masculine singular noun.

In Portuguese:

• masculine singular: o
• feminine singular: a
• masculine plural: os
• feminine plural: as

So:

• o plano
• a ideia
• os planos
• as ideias

You usually need to learn the gender together with the noun.

São is one of the trickier words for English speakers because of the nasal sound.

Roughly:

• s = like s in see
• ão = a nasal sound, something like ow but through the nose

A rough English-friendly approximation is something like sowng without a clear final g, but that is only approximate.

Important point:

• the ã is nasal
• the o is not pronounced like a full English oh

This same -ão ending appears in many common words:

• não
• mão
• irmão

A rough pronunciation guide:

EH-lees sowng KON-tra oo PLA-noo

That said, natural Brazilian pronunciation is usually smoother than that. A few notes:

• Eles often sounds like EH-lis or EH-lees, depending on accent.
• são has the nasal ão sound.
• contra has stress on CON.
• plano has stress on PLA.

So the stressed parts are:

E-les SÃO CON-tra o PLA-no

The normal word order is:

• Eles são contra o plano.

This is the most neutral and natural order.

You can move things around for emphasis, for example:

• Contra o plano, eles são.

But that sounds much less normal in everyday speech and may feel literary or emphatic. For learners, stick with the standard order:

• subject + verb + complement

Yes. It is very useful and very common.

You can use it with many subjects:

• Eu sou contra isso. = I’m against that.
• Você é contra a ideia? = Are you against the idea?
• Nós somos contra a mudança. = We are against the change.
• Elas são contra o projeto. = They are against the project.

So ser contra + noun/pronoun is a great pattern to memorize.

The most common opposite is ser a favor de = to be in favor of.

Examples:

• Eles são contra o plano. = They are against the plan.
• Eles são a favor do plano. = They are in favor of the plan.

Notice that de + o = do:

• a favor de o plano → a favor do plano

This is a useful contrast to learn together:

• ser contra
• ser a favor de