A empresa cujo site ficou fora do ar vai enviar outro link amanhã.

Cujo means whose.

In A empresa cujo site ficou fora do ar..., it connects a empresa and site:

• a empresa = the company
• cujo site = whose website

So the sentence is literally structured like:

• The company whose website went down...

Portuguese uses cujo / cuja / cujos / cujas for this kind of relationship of possession. It is more formal and more precise than trying to use que.

Because cujo agrees with the thing that comes after it, not with the owner.

Here:

• owner: a empresa → feminine
• possessed thing: site → masculine singular

Since site is masculine singular, you use cujo.

Compare:

• a empresa cujo site... = the company whose website...
• a empresa cuja página... = the company whose page...
• as empresas cujos sites... = the companies whose websites...
• as empresas cujas páginas... = the companies whose pages...

So the form depends on the noun after cujo.

Not in good standard Portuguese. That sounds awkward or incorrect.

Learners often try to build whose with something like:

• que ... dela/dele

But in this sentence, standard Portuguese strongly prefers cujo:

• A empresa cujo site ficou fora do ar...

A more colloquial workaround is sometimes possible, such as restructuring the sentence:

• A empresa, e o site dela ficou fora do ar, ... — but this is clumsy and not a good equivalent here.

So for a clean, natural sentence, cujo is the best choice.

Because cujo is not followed by an article.

So you say:

• cujo site
• cuja página
• cujos clientes
• cujas lojas

Not:

• cujo o site
• cuja a página

This is a very important rule. In Portuguese, cujo already includes the relationship of possession, so adding o/a/os/as after it is wrong.

Ficar fora do ar is a very common expression meaning:

• to be offline
• to be down
• to be unavailable

So:

• O site ficou fora do ar = The website went down / The website was offline

Literally, fora do ar means out of the air, but in real usage it means something is not operating, especially:

• websites
• internet services
• TV/radio broadcasts
• systems

Examples:

• O sistema está fora do ar. = The system is down.
• O site ficou fora do ar por duas horas. = The website was down for two hours.

Ficou is the preterite form of ficar, and here it suggests a change of state:

• ficou fora do ar = went offline / ended up offline

This is different from:

• estava fora do ar = was offline
• está fora do ar = is offline

So the differences are:

• ficou fora do ar → focuses on the event/change
• estava fora do ar → describes the state in the past
• está fora do ar → describes the current state

In your sentence, ficou makes sense because it refers to what happened to the website.

Both mean future, but vai enviar is very common in everyday Brazilian Portuguese.

• vai enviar = is going to send / will send
• enviará = will send

The version with ir + infinitive is often more natural in spoken Brazilian Portuguese:

• vai enviar outro link amanhã

The simple future enviará is correct too, but it can sound more formal, more written, or less conversational depending on context.

So:

• A empresa vai enviar outro link amanhã. = very natural
• A empresa enviará outro link amanhã. = also correct, slightly more formal

Outro link means another link or a different link.

It usually suggests that:

• there was already a link before, and
• a new/replacement one will be sent

So in context, the company is probably sending a new link because the original website or access point had a problem.

Yes, amanhã can move around.

In this sentence:

• A empresa cujo site ficou fora do ar vai enviar outro link amanhã.

putting amanhã at the end is very natural.

You could also say:

• Amanhã, a empresa cujo site ficou fora do ar vai enviar outro link.
• A empresa cujo site ficou fora do ar amanhã vai enviar outro link.

But the second version can sound confusing, because it may seem like ficou fora do ar amanhã, which does not make sense.

So the end position is clear and natural because it obviously modifies vai enviar.

Yes. In Brazilian Portuguese, site is usually masculine:

• o site
• um site
• meu site

That is why the sentence has:

• cujo site

If it were a feminine noun, you would use cuja instead.

The sentence breaks down like this:

• A empresa = the company
• cujo site ficou fora do ar = whose website went down
• vai enviar outro link amanhã = is going to send another link tomorrow

So the main clause is:

• A empresa vai enviar outro link amanhã.

And the middle part:

• cujo site ficou fora do ar

is a relative structure giving extra information about a empresa.

Yes, in a very literal grammatical sense, sometimes cujo is close to of which. But in natural English, whose is almost always the best translation here, even for things, not just people.

So:

• A empresa cujo site...
• natural English: The company whose website...

You would not normally say:

• The company of which the website...

That sounds unnatural in English.

So for learning purposes, the best equivalent is usually whose.

It is fairly neutral to slightly formal, mainly because of cujo.

Everything else is very normal everyday Portuguese:

• ficou fora do ar
• vai enviar
• outro link amanhã

But cujo is less common in casual speech than in writing, news, customer service messages, official communication, or more polished language.

So the sentence sounds natural and correct, but a bit more formal than everyday conversation.

A more conversational rewrite might be:

• A empresa, que teve o site fora do ar, vai enviar outro link amanhã.
• A empresa, cujo site saiu do ar, vai enviar outro link amanhã.

Or even:

• A empresa vai enviar outro link amanhã porque o site ficou fora do ar.

These versions avoid cujo, which many speakers use less often in casual conversation.

Still, your original sentence is completely correct and natural, especially in written Portuguese.

In Brazilian Portuguese, cujo is pronounced approximately like:

• KOO-zhoh

More closely:

• cu sounds like koo
• j sounds like the s in measure or the zh sound

So:

• cujo ≈ KOO-zho

The exact pronunciation varies by accent, but that is a good approximation for learners.

No. It is used for several kinds of things that stop working or broadcasting.

Common examples:

• O site ficou fora do ar. = The website went down.
• O sistema ficou fora do ar. = The system went down.
• A rádio ficou fora do ar. = The radio station went off the air.
• O serviço ficou fora do ar. = The service was unavailable.

So it is broader than just websites, though websites are a very common use.