Learn Hardcore Portuguese (Portugal): Hoje à noite vou ficar em casa de pijama a ver um filme. - Tonight I’m going to stay at home in my pyjamas watching a movie.

Vou ficar em casa. – the form vou clearly tells you it’s 1st person singular (I).
Adding eu is only needed for emphasis or contrast:
- Hoje à noite eu vou ficar em casa. (I will stay home, not someone else.)

So the version without eu is the most neutral and natural here.

Why use "vou ficar" instead of "ficarei" for the future?

Both are grammatically correct, but they sound different in everyday use:

vou ficar = ir (present) + infinitive
- Most common in spoken Portuguese.
- Similar to English "I’m going to stay".
- Used for plans, intentions, and near future.
ficarei = synthetic future tense
- More formal or written.
- Can sound stiff or bookish in casual speech.

So "Hoje à noite vou ficar em casa..." is the natural conversational choice.

Could I say "Hoje à noite fico em casa..." with the present tense to talk about the future?

Yes. Present tense in Portuguese can also be used for future events, especially planned or scheduled ones, similar to English "Tonight I stay home."

Hoje à noite vou ficar em casa. – I’m going to stay home (intention/plan).
Hoje à noite fico em casa. – I (definitely) stay home tonight / I’m staying home tonight (sounds somewhat more decided or scheduled).

Both are correct; "vou ficar" is slightly more neutral and very common in speech.

Why is it "em casa" and not "na casa"?

When "casa" means "home" in general, Portuguese normally doesn’t use an article:

em casa – at home
para casa – (going) home
de casa – from home

If you say "na casa" (em + a casa), you’re talking about a specific house as a building:

Ele está na casa do João. – He’s at João’s house (in that particular house).
Vou ficar em casa. – I’m going to stay at home (my place / home in general).

So "vou ficar em casa" is exactly what you want for "I’m going to stay at home."

Why is it "de pijama" for "in pajamas"? Could I say "em pijama"?

In Portuguese, "de" is very commonly used to describe what someone is wearing or their state/appearance:

de pijama – in pajamas
de casaco – wearing a coat
de calças azuis – in blue trousers
de chapéu – with a hat on

So "de pijama" is the most natural expression.

You can say "em pijama", and it will be understood, but "de pijama" is what people normally say in European Portuguese. The article is also dropped here: not *de o pijama, just de pijama.

Why is "pijama" singular in Portuguese when English has "pajamas" (plural)?

It’s mainly a language-specific difference in how the item is conceptualized:

In Portuguese, "o pijama" is treated as one set of sleepwear → singular.
In English, "pajamas" is grammatically plural.

So:

Estou de pijama. – I’m in pajamas.
Onde está o meu pijama? – Where are my pajamas?

Grammatically:

pijama is masculine singular: o pijama, um pijama, meu pijama.
You only use the plural pijamas if you literally mean multiple sets of pajamas.

What is the function of "a" in "a ver um filme"? Why not just "ver um filme"?

Here "a" + infinitive (a ver) is a very typical European Portuguese way to mark a continuous/ongoing action, especially after verbs like estar, ficar, ir, vir:

ficar a ver – to stay watching
estar a ver – to be watching
ir a ver (less common) – to go and watch

So:

vou ficar em casa a ver um filme
≈ I’m going to stay at home watching a film.

If you drop the "a" and say "*vou ficar em casa ver um filme", it sounds wrong or at least very unnatural. You’d have to restructure, for example:

Vou ficar em casa e ver um filme. – I’ll stay home and watch a film (two coordinated actions, not a continuous construction).

In this sentence, "a ver" ties the staying and watching together as one ongoing situation.

Can I say "vendo um filme" instead of "a ver um filme" in European Portuguese?

You can, and people will understand:

...vou ficar em casa, de pijama, vendo um filme.

However, in European Portuguese:

The preferred and most natural way to express the progressive is "estar / ficar / ir + a + infinitive":
- estou a ver, fico a ver, vou ficar a ver.
The gerund (vendo, comendo, falando) is still correct, but:
- It’s much more characteristic of Brazilian Portuguese in this kind of sentence.
- It can sound a bit Brazilian or at least less idiomatic in Portugal in everyday speech.

If you’re learning Portuguese from Portugal, it’s safer and more natural to say:

Hoje à noite vou ficar em casa de pijama a ver um filme.

Would a Brazilian speaker say this sentence differently?

Probably yes. A very natural Brazilian Portuguese version would be:

Hoje à noite vou ficar em casa, de pijama, vendo um filme.
or
Hoje à noite vou ficar em casa, de pijama, assistindo a um filme.

Main differences:

Use of gerund: vendo / assistindo instead of a ver.
Verb choice: assistir (a) um filme is more common in Brazil for “to watch a film”. In Portugal, everyday speech usually prefers ver um filme.
Subject pronoun: Brazilians often include "eu" (Eu vou ficar...), though omitting it is still correct.

The original sentence is clearly European Portuguese, but Brazilians would fully understand it.

Can I change the word order, e.g. "vou ficar de pijama em casa a ver um filme"?

Yes, Portuguese word order is fairly flexible, especially with complements like these. All of these are possible and natural:

Hoje à noite vou ficar em casa de pijama a ver um filme.
Hoje à noite vou ficar de pijama em casa a ver um filme.
Hoje à noite vou ficar em casa a ver um filme de pijama.

The basic meaning doesn’t change; you’re just shifting the focus slightly:

Putting "em casa" earlier makes the place more prominent.
Putting "de pijama" at the end can slightly highlight being in pajamas.

In writing, you’d often add commas for clarity:

Hoje à noite vou ficar em casa, de pijama, a ver um filme.

Why is it "ver um filme" and not "assistir um filme"?

In European Portuguese:

Everyday, natural way to say “watch a film” is:
- ver um filme – to watch/see a film.
assistir is used, but:
- It normally takes the preposition a: assistir a um filme.
- It sounds more formal or is used for events and performances:
  - assistir a um espetáculo, assistir a uma aula.

In Brazilian Portuguese:

assistir (a) um filme, assistir à TV is very common in speech.
ver um filme also exists, but assistir is very frequent for “to watch”.

So, in Portugal, "ver um filme" is exactly what you want in a casual sentence like this.