Learn Hardcore Portuguese (Portugal): Quando há penálti, todos os adeptos ficam em silêncio por um segundo. - When there is a penalty, all the fans stay silent for a second.

Há penálti = There is a penalty (situation has occurred).
Há um penálti is grammatically correct, but in this context it sounds less idiomatic, more like there is a penalty kick as an isolated object, not as a referee decision.
É penálti is also heard in stadiums and on TV, usually as a reaction: That’s a penalty! (judging a specific play). It feels a bit more evaluative/subjective than há penálti, which sounds more like a neutral description of the situation.

So the sentence uses the very natural, set expression for “a penalty has been given/there is a penalty situation”: Quando há penálti…

What exactly does há mean here, and why does it have that accent and a silent h?

Há is the 3rd person singular of haver used in an impersonal way, and here it means there is / there are.

Key points:

In European Portuguese, há is the normal way to say there is / there are:
- Há penálti. = There is a penalty.
- Há muitos adeptos. = There are many fans.
The h in há is always silent in Portuguese.
The accent (á) does two things:
- It marks the stressed syllable: you pronounce the whole word as [á], like “ah” in English, but stronger and a bit more open.
- It distinguishes há (from haver) from a (the preposition or article), which are pronounced the same but have different functions.

So há ≈ there is / there are and is pronounced just as á, without any [h] sound.

Why is it há and not tem for “there is a penalty”?

This is a European Portuguese sentence, and in Portugal:

Há is the normal, standard way to say there is / there are.
Using tem to mean there is / there are is typical of Brazilian Portuguese in everyday speech.

So:

In Portugal:
- Há penálti. (most natural)
In Brazil, you can hear both:
- Tem pênalti.
- Há pênalti. (more formal/written)

Since the sentence is in Portuguese from Portugal, há penálti is exactly what you expect.

What does penálti mean, and why is it written like that?

Penálti is the European Portuguese adaptation of the English word penalty (in the football sense).

It keeps roughly the same pronunciation but is spelled according to Portuguese spelling rules:
- Final -y becomes -i.
- The accent á marks the stressed syllable: pe-NÁL-ti.
It’s a masculine noun:
- o penálti = the penalty (kick)
- plural: os penáltis

In this sentence there’s no article (o), because in football jargon it’s very common to talk about decisions without articles:

Há penálti. = There is a penalty.
Há falta. = There is a foul.
Há fora-de-jogo. = There is offside.

You could say há um penálti, but the version without the article is the typical “referee decision” style.

What does adeptos mean here? Is it the same as “fans”?

Yes. In this context, os adeptos means the supporters / the fans of a team.

Singular: o adepto = supporter, fan
Plural: os adeptos = supporters, fans

In European Portuguese sports language:

adeptos is the normal word for fans of a club or national team.
fãs (from English fans) is also used, but more generally (music fans, movie fans, etc.), and in football you’ll hear adeptos a lot.
In Brazilian Portuguese you’d normally hear torcedores instead of adeptos.

So todos os adeptos here is “all the supporters / all the fans in the stadium.”

Why is the verb plural (ficam) in todos os adeptos ficam?

Because the subject is plural.

Subject: todos os adeptos (all the supporters).
This is clearly more than one person.
So the verb must be in the 3rd person plural: ficam.

If the subject were singular, the verb would also be singular:

Todo o adepto fica em silêncio.
Every supporter goes quiet / remains silent.

What does ficam mean here? Does ficar mean “to become” or “to stay”?

Ficar can mean several things, including:

To become / to get (change of state)
- Fico nervoso. = I get nervous / I become nervous.
To stay / to remain (state doesn’t change)
- Fico em casa. = I stay at home.
To be located
- O estádio fica em Lisboa. = The stadium is (located) in Lisbon.

In todos os adeptos ficam em silêncio, it’s mainly sense (1): to become / to go quiet:

ficar em silêncio ≈ to go quiet / to fall silent
but it also overlaps with (2): to remain in silence, for that moment.

So ficam em silêncio is best understood as go quiet / fall silent (and stay that way briefly).

Why is it ficam em silêncio and not ficam silenciosos?

Both are grammatically correct, but they’re not used in exactly the same way:

ficar em silêncio is a very common, idiomatic expression meaning:
- to go quiet / to fall silent
- to be in a state of silence, not making any noise
ficar silenciosos (literally become silent) is also possible, but:
- em silêncio sounds more natural when describing a group suddenly going quiet.
- silencioso as an adjective can sometimes focus more on the characteristic (“a quiet person/neighborhood”) rather than this brief, dramatic silence in a stadium.

So ficam em silêncio is the usual, idiomatic way to describe this short, collective silence.

What is the function of por in por um segundo? Could I say durante um segundo instead?

Here por introduces the duration of the silence:

por um segundo = for one second

You can also say durante um segundo; it’s perfectly correct and means essentially the same thing.

Nuance:

por um segundo is very common and feels a bit lighter and more conversational.
durante um segundo may sound slightly more formal or a bit more emphasized, but in everyday speech they are often interchangeable in this type of sentence.

So both:

Todos os adeptos ficam em silêncio por um segundo.
Todos os adeptos ficam em silêncio durante um segundo.

are good European Portuguese.

Why is it um segundo and not uma segunda?

Because segundo here is the noun for the unit of time (a second), and that noun is masculine in Portuguese:

um segundo = one second
dois segundos = two seconds

Segunda (feminine) is used in other meanings, for example:

segunda-feira = Monday
a segunda vez = the second time (ordinal number, feminine because vez is feminine)

In this sentence we’re clearly talking about a time span (one second), so it must be um segundo.

Could I move the quando-clause to the end? Is the comma necessary?

Yes, you can change the order:

Original: Quando há penálti, todos os adeptos ficam em silêncio por um segundo.
Also possible: Todos os adeptos ficam em silêncio por um segundo quando há penálti.

Both versions are natural in European Portuguese.

About the comma:

When the quando-clause comes first, you normally use a comma:
- Quando há penálti, todos os adeptos…
When it comes second, the comma is usually not written:
- …ficam em silêncio por um segundo quando há penálti.

So the comma you see is just standard punctuation for a subordinate clause at the beginning.

Why is it quando há and not quando houver, even though this can refer to future situations?

Both forms exist, but they’re used in different contexts:

Quando há penálti, todos os adeptos ficam em silêncio por um segundo.
Uses present indicative (há, ficam).
This describes a general, habitual fact: Whenever there is a penalty, this is what happens.
Quando houver penálti, avisa-me.
Uses future subjunctive (houver).
This talks about a specific future event or condition: When there is a penalty (in the future), let me know.

In your sentence, we are stating a general rule about what typically happens at matches in general, so quando há with the present indicative is exactly right.