Learn Hardcore Portuguese (Portugal): Mal o filme começa, a sala fica em silêncio. - As soon as the film starts, the room becomes silent.

It does not mean “badly” here. It’s introducing a time clause: the moment the film starts, X happens.
A close English equivalent in tone is: “Hardly has the film started when the room goes silent.”

What’s the difference between mal and mau?

They’re different words:

mal (with l)
- Usually an adverb or conjunction
- Meanings: badly, poorly, hardly, barely, as soon as
- Examples:
  - Ele canta mal. – He sings badly.
  - Mal cheguei, ele saiu. – As soon as I arrived, he left.
mau (with u)
- An adjective (masculine form of má)
- Meaning: bad, evil (describing a noun)
- Examples:
  - um filme mau – a bad film
  - Ele é mau. – He is bad/mean.

In your sentence it must be mal because it’s linking two actions in time (as soon as), not describing a noun.

How do I know that mal here means “as soon as” and not “badly”?

Look at its position and function:

It comes at the beginning of the clause.
It is immediately followed by a subject and a verb: Mal o filme começa…
It’s linking this clause with another one: …, a sala fica em silêncio.

When mal is used like this – at the start of a clause, before a subject + verb, and followed by another clause – it’s acting as a temporal conjunction (“as soon as / hardly … when”), not as “badly”.

If it meant “badly”, you would see a different structure, something like:

O filme começa mal. – The film starts badly.
(here mal directly modifies the verb)

Can I replace mal with quando, assim que, or logo que?

Yes, with small differences in nuance:

Quando o filme começa, a sala fica em silêncio.
– quando = when (neutral, very common)
Assim que o filme começa, a sala fica em silêncio.
– assim que = as soon as (very close to mal, very natural)
Logo que o filme começa, a sala fica em silêncio.
– logo que = also as soon as (common in Portugal)
Mal o filme começa, a sala fica em silêncio.
– mal = hardly / barely / as soon as and often suggests the second action follows immediately, almost overlapping the first.

All are correct. Mal, assim que, and logo que are the closest to the English “as soon as”; mal adds a slight flavour of “barely started and already…”.

Is the word order Mal o filme começa fixed, or can I say Mal começa o filme?

Both are grammatically correct:

Mal o filme começa, a sala fica em silêncio.
(conjunction + subject + verb)
Mal começa o filme, a sala fica em silêncio.
(conjunction + verb + subject)

In European Portuguese, both orders are natural. The version with Mal o filme começa is slightly more neutral and clearer for learners, because it keeps the subject (o filme) right after mal, showing clearly that mal is introducing that whole clause (as soon as the film starts).

Why are the verbs in the present tense (começa, fica) if we might translate it with the English past (“started, fell silent”)?

Portuguese often uses the simple present to talk about:

Habitual/general facts:
- Mal o filme começa, a sala fica em silêncio.
  → Every time the film starts, the room goes quiet.
A vivid retelling of a specific scene (historical/dramatic present):
- Used in storytelling: you “see” it happening now.

If you are describing one specific event in the past, you’d more likely say:

Mal o filme começou, a sala ficou em silêncio.
→ As soon as the film started, the room fell silent.

For a future event, you could say:

Quando / Assim que / Logo que o filme começar, a sala ficará em silêncio.
(começar is future subjunctive; ficará is simple future)

Could I say Mal o filme começar, a sala ficará em silêncio instead?

Yes, and that sounds like a more explicitly future-oriented, slightly more formal version:

Mal o filme começar, a sala ficará em silêncio.

Here:

começar is the future subjunctive (same form as the infinitive in this verb):
mal (ele) começar = as soon as (it) starts (in the future).
ficará is the simple future.

So:

Mal o filme começa, a sala fica em silêncio.
→ Describes a general/habitual fact, or a vivid present narration.
Mal o filme começar, a sala ficará em silêncio.
→ A prediction about a future situation (e.g. describing how it will be in a new cinema).

Why is there a comma before a sala fica em silêncio?

Because we have two clauses:

Mal o filme começa – a subordinate time clause (when/as soon as the film starts)
a sala fica em silêncio – the main clause (the result)

In European Portuguese writing, it’s normal to separate the subordinate clause from the main clause with a comma when the subordinate clause comes first:

Mal o filme começa, a sala fica em silêncio.
Quando chego a casa, janto.
Assim que termina a aula, ele vai embora.

What does ficar mean here in a sala fica em silêncio?

Here ficar means “to become / to end up / to go into a state and stay there (for a while)”, not “to stay at a place”.

a sala fica em silêncio
→ the room becomes / falls silent and remains that way (for some time)

Compare:

A sala está em silêncio. – The room is silent. (describes the current state)
A sala fica em silêncio. – The room becomes/goes silent. (focuses on the change into that state)

So ficar + state (adjective or expression like em silêncio) often describes a change of state:

ficar triste – to become sad
ficar escuro – to get dark
ficar em silêncio – to fall silent

Why do we say em silêncio and not something like silenciosa?

Both are possible, but they’re not identical:

A sala fica em silêncio.
- Very idiomatic, very common.
- Literal: the room ends up in silence.
- Emphasises the absence of sound, the hush that falls over the room.
A sala fica silenciosa.
- Also correct.
- Uses the adjective silenciosa (silent).
- Slightly more descriptive/neutral; does not have as strong a set-expression feel as ficar em silêncio.

Em silêncio is part of a very common fixed expression:

ficar / ficar em silêncio – to be / remain / fall silent
ficar em completo silêncio – to be in complete silence

That’s why a sala fica em silêncio sounds especially natural here.

Why is it a sala and not something like na sala or o cinema?

Here a sala is the subject of the sentence, not a location:

a sala fica em silêncio
→ the room (subject) + ficar em silêncio (verb + complement)

If you said na sala, you’d be giving a location for some other subject:

Na sala, as pessoas ficam em silêncio.
→ In the room, the people fall silent.

As for o cinema, in European Portuguese:

o cinema can mean the building, the place, or even the institution.
a sala (in a cinema context) is the screening room / auditorium.

So:

Mal o filme começa, a sala fica em silêncio.
→ Focus on the auditorium itself going silent (everyone inside it).

You could say Mal o filme começa, o cinema fica em silêncio, but that tends to sound more like “the whole cinema” (the establishment) goes quiet, which is a bit less precise than focusing on the room where the film is playing.