É importante que o banheiro seja limpo antes da festa.

Because é importante que... is a very common Portuguese pattern meaning it is important that...

The word que introduces the second clause:

• É importante = It is important
• que o banheiro seja limpo = that the bathroom be cleaned / that the bathroom is cleaned

In Portuguese, expressions like é importante, é necessário, é bom, é melhor, and é possível are often followed by que plus the subjunctive.

Seja is the present subjunctive form of ser.

It appears here because é importante que... normally triggers the subjunctive. Portuguese uses the subjunctive after expressions of:

• importance
• necessity
• doubt
• emotion
• desire
• possibility

So:

• É importante que o banheiro seja limpo. not
• É importante que o banheiro é limpo. ❌

To an English speaker, this can feel similar to formal English It is important that the bathroom be cleaned.

Seja is the present subjunctive of ser for:

• eu
• você
• ele/ela
• a gente

In this sentence, the subject is o banheiro, which is third person singular, so we use seja.

A few forms of ser in the present subjunctive are:

• que eu seja
• que você seja
• que ele/ela seja
• que nós sejamos
• que vocês sejam
• que eles/elas sejam

The verb limpar has two participle forms in practice:

• limpado (regular participle)
• limpo (irregular participle, very common)

After ser in a passive construction, Portuguese strongly prefers the irregular participle limpo:

• O banheiro foi limpo. = The bathroom was cleaned.
• É importante que o banheiro seja limpo. = It is important that the bathroom be cleaned.

So seja limpo is the natural form here.

Here it is best understood as part of a passive construction:

• o banheiro seja limpo = the bathroom be cleaned

That said, participles in Portuguese often behave a lot like adjectives, which is why limpo agrees in gender and number with banheiro:

• banheiro → masculine singular
• therefore limpo

Compare:

• o banheiro seja limpo
• a cozinha seja limpa
• os banheiros sejam limpos
• as cozinhas sejam limpas

Because it must agree with the noun it refers to.

Here, banheiro is:

• masculine
• singular

So we use limpo.

Other possibilities:

• o quarto seja limpo
• a sala seja limpa
• os quartos sejam limpos
• as salas sejam limpas

This kind of agreement is very common in Portuguese.

Portuguese uses definite articles much more often than English.

So o banheiro often sounds more natural than just banheiro, especially when referring to a specific bathroom already understood in context:

• É importante que o banheiro seja limpo...

Without the article, it would usually sound less natural here.

This is a very useful distinction.

• seja limpo focuses more on the action/process of cleaning, or a passive idea: be cleaned
• esteja limpo focuses more on the resulting state/condition: be clean

So:

• É importante que o banheiro seja limpo antes da festa.
  = It is important that the bathroom be cleaned before the party.

• É importante que o banheiro esteja limpo antes da festa.
  = It is important that the bathroom be clean before the party.

In real life, both can work, but they emphasize slightly different things.

Because de + a usually contracts to da in Portuguese.

So:

• antes de + a festa → antes da festa

This is a normal contraction:

• de + o = do
• de + a = da
• de + os = dos
• de + as = das

So antes da festa simply means before the party.

No. This is an impersonal expression.

Portuguese often uses structures like:

• É importante...
• É necessário...
• É possível...
• É melhor...

These work like English It is important..., where it does not refer to any real thing. In Portuguese, there is no need to include a word equivalent to that it.

So:

• É importante que o banheiro seja limpo. literally works as:
• Is important that the bathroom be cleaned.

But in natural English, we say:

• It is important that the bathroom be cleaned.

Yes, that structure is possible, and learners will encounter it.

• É importante que o banheiro seja limpo...
• É importante o banheiro ser limpo...

Both can express a similar idea. However, the version with que + subjunctive is very common and is often the one taught first because it fits a major Portuguese grammar pattern.

The que + subjunctive version usually sounds more standard and natural in many contexts.

In passive constructions, Portuguese uses ser:

• foi limpo
• é limpo
• seja limpo

That is different from estar, which usually describes a state:

• está limpo = is clean

So:

• ser limpo = to be cleaned / to be cleaned as a passive action
• estar limpo = to be clean

That is why seja limpo makes sense in this sentence.

It sounds fairly neutral to slightly formal, mainly because of the subjunctive structure and the passive phrasing.

In everyday speech, many Brazilians might also say something simpler, depending on context, such as:

• É importante limpar o banheiro antes da festa.
• É importante que limpem o banheiro antes da festa.
• O banheiro tem que estar limpo antes da festa.

But the original sentence is absolutely correct and natural. It just sounds a bit more polished or formal than the most casual everyday version.