Learn Hardcore Portuguese (Portugal): O estádio enche-se de adeptos quando há jogo importante. - The stadium fills up with fans when there is an important match.

encher alone is normally “to fill (something)”:
- Eu encho o copo. – I fill the glass.
encher-se de adeptos = “to fill up with fans / to get full of fans” (the stadium itself becomes full).

So O estádio enche-se de adeptos is best understood as:

The stadium fills up with supporters / The stadium gets full of supporters.

Without -se, O estádio enche de adeptos sounds incomplete or odd in European Portuguese; with this verb, the structure encher-se de + noun is the natural one to express “to get full of …”.

Could I put se before the verb and say O estádio se enche de adeptos?

In European Portuguese, in a normal affirmative sentence like this, the clitic pronoun usually comes after the verb (enclisis):

O estádio enche-se de adeptos. ✅ (European Portuguese standard)
O estádio se enche de adeptos. ❌ (sounds Brazilian to Europeans)

In Brazilian Portuguese, O estádio se enche de adeptos would be normal, because Brazilian Portuguese strongly prefers se before the verb (proclisis).

So:

European Portuguese: enche-se
Brazilian Portuguese: se enche (in this kind of sentence)

Why is it enche-se de adeptos and not enche-se com adeptos?

With encher(-se), European Portuguese strongly prefers de to indicate what something is filled with:

encher-se de adeptos – to get filled with supporters
encher-se de água – to fill with water
encher-se de gente – to get full of people

Using com here (enche-se com adeptos) is possible but sounds less idiomatic and more foreign-influenced. The fixed, natural pattern is:

encher(-se) de + noun = “to fill / to fill up with …”

What exactly does adeptos mean here? Is it just “fans”?

Adeptos in this context means sports supporters, especially of a football (soccer) team.

adeptos = supporters, fans (of a club, team, or cause)
In stadium/football context in Portugal, adeptos is the default word.

Some nuances:

adeptos – normal, neutral word for supporters of a team.
fãs – more like “fans” of people, bands, celebrities, etc. (also used for sports, but adeptos sounds more traditionally football-related in Portugal).
In Brazil, you’d more often hear torcedores for football fans.

So here adeptos is “the people who support the team and go to the stadium”.

Why is there no article before jogo importante? Why not quando há um jogo importante?

Both are grammatically possible, but there is a nuance:

quando há jogo importante
– very generic and habitual: “whenever there is an important match (in general)”.
quando há um jogo importante
– still general, but feels a bit more like referring to concrete individual events (“when there is an important match” on a given day).

With haver in the “there is/are” sense, Portuguese often drops the article for generic or habitual statements:

Há escola amanhã. – There is school tomorrow.
Há trânsito à tarde. – There is traffic in the afternoon.
Quando há jogo importante, o estádio enche-se… – When there’s an important match, the stadium fills up…

So sem article here reinforces a broad, habitual rule: on any occasion when there is an important match.

What does há mean here, and how is it different from tem or existe?

In this sentence há is the verb haver meaning “there is / there are”:

há jogo importante = there is an important match.

Differences:

há – standard, formal verb for existence in both Portugal and Brazil.
tem – in Brazilian Portuguese, very commonly used in speech as “there is/are”:
Tem jogo hoje. – There’s a match today.
In European Portuguese, using tem like this is much rarer and sounds Brazilian.
existe / existem – literally “exists / exist”, more formal and less common in everyday speech for this type of sentence.

So in European Portuguese, há is the natural choice here:

quando há jogo importante – when there is an important game.

Does há here have anything to do with the meaning “ago” (like “há dois anos”)?

No. Haver has two common uses:

Existence – “there is / there are”
- Há jogo importante. – There is an important match.
Time ago – “ago”
- Há dois anos. – Two years ago.

In your sentence, it’s the existence use: “there is an important match”, not “ago”.

Why is it quando há jogo importante and not quando tiver jogo importante for the future?

Portuguese often uses the present tense with quando to express habitual situations and also future events that are seen as regular or predictable.

Quando há jogo importante, o estádio enche-se de adeptos.
= Whenever there’s an important game, the stadium fills up with supporters. (habitual rule)

For a specific future event, you could indeed use the future of the subjunctive:

Quando houver um jogo importante, o estádio vai encher-se de adeptos.
= When there is an important game (in the future), the stadium will fill up with supporters.

In your sentence, the idea is general habit, so the present há / enche-se is perfect.

Can I change the word order and say Quando há jogo importante, o estádio enche-se de adeptos?

Yes, both orders are correct:

O estádio enche-se de adeptos quando há jogo importante.
Quando há jogo importante, o estádio enche-se de adeptos.

Differences:

Starting with O estádio puts the focus first on the stadium.
Starting with Quando há jogo importante highlights the condition/time first.

In writing, when the quando-clause comes first, it is normally followed by a comma.

Can I omit de adeptos and just say O estádio enche-se quando há jogo importante?

Yes, that is grammatically correct:

O estádio enche-se quando há jogo importante.
= The stadium gets full when there’s an important game.

The difference:

enche-se alone focuses on the idea “it becomes full”.
enche-se de adeptos specifies what it fills up with (supporters).

In most real contexts about stadiums, de adeptos adds useful information, so it’s typically included.

Could I also say O estádio fica cheio de adeptos quando há jogo importante? What’s the difference?

Yes, you can:

O estádio enche-se de adeptos…
literally focuses on the process of filling up (“fills up with supporters”).
O estádio fica cheio de adeptos…
focuses more on the resulting state (“ends up full of supporters”).

In everyday usage, both describe almost the same situation.
Encher-se sounds slightly more dynamic; ficar cheio sounds slightly more like describing the final condition.

How do I pronounce O estádio enche-se de adeptos naturally in European Portuguese?

Approximate pronunciation (European Portuguese):

O estádio – /u ish-TÁ-diu/
enche-se – /ÊN-shə-sə/ (the final -se is very reduced, almost like “sə”)
de adeptos – /dɨ ɐ-DÉP-tush/

Spoken together, it flows roughly as:

u ish-TÁ-diu ÊN-shə-sə dɨ ɐ-DÉP-tush

Key points:

estádio has the stress on -tá- (ish-TÁ-diu).
The vowel in de is very reduced: /dɨ/.
Final -os in adeptos is often pronounced like -ush /uʃ/ in European Portuguese.