Learn Hardcore Portuguese (Portugal): Hoje estou prestes a sair de casa, mas ainda estou de pijama. - Today I am about to leave the house, but I am still in my pyjamas.

estou prestes a sair de casa – I am about to leave the house (temporary state: being on the point of leaving)
ainda estou de pijama – I am still in my pyjamas (temporary state: being in pyjamas)

In English, you also repeat “I’m”: “I’m about to leave home, but I’m still in my pyjamas.”
So the structure is actually very parallel; Portuguese just makes it slightly more explicit that these are two simultaneous states.

What exactly does prestes a mean? Is it like “about to”, “almost”, or something else?

Prestes a + infinitive is a set expression meaning “about to”, “on the point of” doing something.

estou prestes a sair – I’m about to leave
o filme está prestes a começar – the film is about to start

It implies:

the action has not started yet, and
you are very close in time to starting it.

It is stronger than a vague future like vou sair (I’m going to leave); estou prestes a sair means almost now.

Can I leave out estou and just say “Hoje prestes a sair de casa…”?

No. Prestes practically always needs a verb, most commonly estar, to form the structure:

estar prestes a + infinitive

So you normally say:

estou prestes a sair
estava prestes a sair
estivemos prestes a desistir

Without estar, prestes sounds incomplete or wrong in modern usage.

Why is it sair de casa and not sair da casa?

In European Portuguese, when casa means “home” (your own place, not just any house), you usually use it without an article and with de:

sair de casa – leave home
chegar a casa – arrive home
estar em casa – be at home

If you say da casa, you are referring to a specific house as a building, often one mentioned earlier:

sair da casa branca – leave the white house
sair da casa do João – leave João’s house

In your sentence, the idea is “leave home”, so de casa is the natural choice.

Could I just say “sair” without “de casa”?

You could, but it would change the meaning or make it less clear.

estou prestes a sair – I’m about to go out / leave (somewhere)
→ context would have to tell you where you’re leaving from.
estou prestes a sair de casa – I’m about to leave home / leave the house
→ explicitly from home.

Since the key contrast in the sentence is about to leave home vs. still in pyjamas, de casa adds important information.

Why is it de pijama and not em pijama? Are both correct?

Both de pijama and em pijama exist, and both can mean “in pyjamas”, but there is a tendency:

de pijama – very common, neutral, slightly more idiomatic here.
em pijama – also correct; can sound a bit more literal or descriptive.

In European Portuguese, de + clothing is very frequent to describe what someone is wearing:

estar de calças de ganga – be in jeans
ir de casaco – go wearing a jacket
ficar de pijama em casa – stay at home in pyjamas

So ainda estou de pijama is the most natural everyday phrasing.

What does ainda add here? Could I say “mas estou de pijama” without ainda?

You can say mas estou de pijama, but ainda adds the idea of “still / yet”:

mas estou de pijama – but I’m in my pyjamas (simple statement)
mas ainda estou de pijama – but I’m still in my pyjamas (I haven’t changed yet, even though you might expect I would have)

So ainda introduces a sense of expectation not yet fulfilled, which matches the contrast: about to leave vs. still in pyjamas.

Can ainda go in a different position, like “mas estou ainda de pijama”?

Yes, you can say:

mas ainda estou de pijama
mas estou ainda de pijama

Both are grammatically correct. The most natural, everyday order is:

mas ainda estou de pijama

Putting ainda after estou (estou ainda de pijama) is possible but sounds a bit more formal or marked in this kind of casual sentence.

Why is it hoje estou… at the beginning? Could I move hoje?

Yes, adverbs like hoje are quite flexible. All of these are possible:

Hoje estou prestes a sair de casa, mas ainda estou de pijama.
Estou hoje prestes a sair de casa, mas ainda estou de pijama.
Estou prestes a sair de casa hoje, mas ainda estou de pijama.

Differences:

Hoje at the beginning is the most natural in speech; it sets the time frame straight away.
Estou hoje… is more marked or literary.
…sair de casa hoje can emphasize today as the day of leaving, like a plan for today rather than a general habit.

In normal conversation, the original order with hoje at the start is the most common.

What is the difference between “Estou prestes a sair de casa” and “Vou sair de casa”?

Both talk about a future action, but:

estou prestes a sair de casa
→ I’m about to leave home
→ leaving is imminent, almost now.
vou sair de casa
→ I’m going to leave home (at some point, maybe soon, maybe later)
→ indicates intention / plan, but not necessarily right now.

So prestes a focuses on nearness in time, while vou focuses on intention.

Is estou prestes a sair de casa a European Portuguese structure, or is it also used in Brazilian Portuguese?

Estar prestes a + infinitive is used in both European and Brazilian Portuguese with the same meaning (“to be about to do something”).

The main European vs. Brazilian difference in talking about ongoing actions is things like:

PT-PT: estou a sair – I’m leaving / I’m in the process of leaving
PT-BR: estou saindo – same meaning

But estou prestes a sair is common and natural in both varieties, and in both it means I’m about to leave (but haven’t started yet).

Could I change the word order to “Hoje estou de pijama, mas estou prestes a sair de casa”? Does that sound different?

Yes, that is correct and natural:

Hoje estou de pijama, mas estou prestes a sair de casa.

The difference is mainly what you emphasize first:

Original: Hoje estou prestes a sair de casa, mas ainda estou de pijama.
→ starts with the idea of about to leave, then adds the contrast: but still in pyjamas.
Changed: Hoje estou de pijama, mas estou prestes a sair de casa.
→ starts with the state in pyjamas, then adds the surprise: but actually about to leave.

Both are fine; you choose based on what you want to highlight first.