A sala de ciências cuja porta estava fechada abre às oito.

Cuja means whose.

So A sala de ciências cuja porta estava fechada... literally works like The science room whose door was closed...

Its job is to connect:

• sala de ciências = the room
• porta = the door

and show that the door belongs to the room.

Because cujo/cuja/cujos/cujas agrees with the noun that comes right after it, not with the earlier noun.

Here, the next noun is porta, which is:

• feminine
• singular

So you need cuja.

Compare:

• cujo livro = whose book
• cuja porta = whose door
• cujos livros = whose books
• cujas portas = whose doors

Both, in different ways:

• It points back to sala as the thing being described.
• But it agrees in gender and number with porta, the possessed noun.

So in A sala de ciências cuja porta...:

• antecedent: sala de ciências
• possessed noun: porta

That is exactly how cujo-words work in Portuguese.

Because standard Portuguese does not use an article after cujo/cuja/cujos/cujas.

So:

• cuja porta = correct
• cuja a porta = incorrect in standard grammar

You can think of cuja as already containing the idea of whose, so Portuguese does not add a or o after it.

Not directly.

This is correct:

• A sala de ciências cuja porta estava fechada...

But this is not correct:

• A sala de ciências que a porta estava fechada...

The problem is that que by itself does not show possession.

If you want to avoid cuja, you usually need to rephrase. For example:

• A sala de ciências com a porta fechada...
• A sala de ciências que estava com a porta fechada...

In everyday Brazilian Portuguese, people often avoid cujo/cuja and use a different structure like that.

It is common in:

• writing
• formal speech
• school grammar
• news or academic style

But in casual spoken Brazilian Portuguese, many people avoid it and use simpler alternatives.

So the sentence is perfectly correct, but it sounds somewhat more formal or written than everyday conversation.

Because fechada is describing porta, and porta is feminine singular.

So the adjective must match:

• porta fechada
• porta aberta
• janela fechada

If the noun were masculine, it would change:

• portão fechado
• livro fechado

Here:

• estava = was
• fechada = closed

Together, estava fechada means was closed.

Because the sentence is combining two different time ideas:

• estava fechada describes a past state of the door
• abre às oito gives the room’s present or usual opening time

That kind of tense mix is possible if the speaker has a reason to mention a past detail about the room while stating a current schedule.

So the structure is grammatical, even though it may feel a little unusual without wider context.

If everything were in the present, you might say:

• A sala de ciências cuja porta está fechada abre às oito.

If everything were in the past, you might say:

• A sala de ciências cuja porta estava fechada abria às oito.

They are not the same.

• abre às oito = it opens at eight
• está aberta às oito = it is open at eight

So abre às oito focuses on the moment the room opens.

Example:

• A loja abre às oito. = The store opens at eight.
• A loja está aberta às oito. = The store is open at eight.

In your sentence, abre às oito means eight is the opening time.

Because às is a contraction of:

• a
  • as

In time expressions, Portuguese normally uses this contraction:

• à uma
• às duas
• às oito

So:

• abre às oito = opens at eight

The grave accent marks crase, the merger of those two vowels.

Sala de ciências means something like:

• science room
• science classroom
• science lab

The structure is:

• sala = room
• de ciências = for science / of science

Portuguese often uses ciências in the plural when referring to the school subject or field in a general way. So sala de ciências is a normal expression.

Yes.

Without commas:

• A sala de ciências cuja porta estava fechada abre às oito.

This is restrictive. It identifies which science room you mean: the one whose door was closed.

With commas:

• A sala de ciências, cuja porta estava fechada, abre às oito.

This is non-restrictive. It sounds like the speaker already knows which science room is being discussed, and cuja porta estava fechada is just extra information about it.

So the commas change the function of the relative clause.

No. In Portuguese, cujo/cuja can refer to people, animals, things, places, institutions, and so on.

So it is completely normal to say:

• a casa cuja janela quebrou
• o livro cujo autor ganhou o prêmio
• a sala cuja porta estava fechada

English learners sometimes think whose is mainly for people, but Portuguese cujo is very comfortable with non-human nouns too.