Learn Hardcore Portuguese (Portugal): O apito do árbitro é tão alto que assusta algumas crianças. - The referee’s whistle is so loud that it scares some children.

apitar – to blow a whistle / to whistle (esp. with a whistle)
- O árbitro apitou. = The referee blew the whistle.
assobiar – to whistle with your mouth
- Ele assobiou uma canção. = He whistled a song.

So here, O apito do árbitro = The referee’s whistle (or The whistle of the referee).

Why is it do árbitro and not de árbitro? What is this do?

do is the contraction of de + o (of + the).

de = of / from
o = the (masculine singular)
de + o → do

So:

o apito do árbitro = the whistle of the referee / the referee’s whistle

If you said de árbitro (without o), it would sound like you’re describing a type or role:

trabalhar de árbitro = to work as a referee

But here you want possession (“the referee’s whistle”), so you need de + o árbitro → do árbitro.

Is árbitro the only word for “referee”? What about juiz?

In European Portuguese, árbitro is the normal word for referee, especially in sports (football, basketball, etc.).

o árbitro = the referee

juiz literally means judge (in court). It can be used in some sports contexts, but for most team sports, árbitro is the standard and sounds more natural.

Why is it é tão alto and not está tão alto? What’s the difference here?

Both ser (é) and estar (está) can be used with adjectives, but they give a different nuance:

é tão alto – describes a characteristic, something seen as more permanent or typical
- The whistle is (in general) so loud that it scares some children.
está tão alto – describes a temporary state or a particular situation
- The whistle is (right now) so loud (maybe because he’s blowing especially hard today).

So O apito do árbitro é tão alto... suggests the referee’s whistle is generally that loud.
You could say está tão alto if you were focusing on a particular moment or unusual loudness.

I thought alto meant “tall”. Why is it used to mean “loud” here?

alto can mean both tall and loud, depending on context. This is similar to English “high”:

um edifício alto = a tall building
uma pessoa alta = a tall person
um som alto = a loud sound (literally “high” volume)

In é tão alto referring to o apito (a sound), the natural interpretation is so loud.
Native speakers very often say:

Fala mais alto. = Speak louder.
A música está muito alta. = The music is very loud.

What does the structure tão ... que mean?

tão ... que means “so ... that” (expressing result or consequence).

Structure:

tão + adjective + que + clause

Examples:

É tão alto que assusta algumas crianças.
= It is so loud that it scares some children.
Ela está tão cansada que não consegue ler.
= She is so tired that she can’t read.

Compare this with:

tão ... como = as ... as (comparison, not result)
- É tão alto como o outro. = It is as loud as the other one.

Why is it assusta and not assustam? Doesn’t it refer to crianças (plural)?

The verb agrees with the subject, not with the object.

In this sentence:

Subject: O apito do árbitro (singular)
Verb: assusta (3rd person singular of assustar)
Object: algumas crianças (some children, plural)

So the agreement is:

O apito do árbitro assusta ... = The whistle (it) scares ...

If the subject were plural, the verb would be plural:

Os apitos do árbitro assustam algumas crianças.
= The referee’s whistles scare some children.

What’s the difference between assustar and assustar-se?

Both exist and are common, but they focus on different sides of the action:

assustar [alguém] = to scare someone (cause fear)
- O apito assusta algumas crianças.
  = The whistle scares some children.
assustar-se = to get scared (the person experiences fear)
- Algumas crianças assustam-se com o apito.
  = Some children get scared by the whistle.

So:

assustar: what causes fear → subject is the cause (the whistle).
assustar-se: who feels fear → subject is the person/animal getting scared.

Why do we say algumas crianças and not just crianças or as crianças?

Each choice changes the meaning slightly:

algumas crianças = some children (an indefinite subset, not all)
- assusta algumas crianças = it scares some (but not all) children.
as crianças = the children (specific/known group, or children in general as a whole)
- assusta as crianças = it scares the children (all of them, or children in general).
crianças without article is more general, like “children” in a generic sense, but with this specific verb and context you’d usually pick algumas or as.

In this sentence, algumas highlights that only some children are scared.

Could I change the word order, like O apito do árbitro assusta algumas crianças porque é tão alto?

Yes, that’s perfectly correct and natural:

O apito do árbitro é tão alto que assusta algumas crianças.
= The whistle is so loud that it scares some children.
O apito do árbitro assusta algumas crianças porque é tão alto.
= The whistle scares some children because it is so loud.

The first one (tão ... que) focuses on the degree (“so loud that ...”).
The second one uses porque (“because”) and focuses more on the reason. Both are fine.

How do you pronounce árbitro and apito in European Portuguese?

Approximate European Portuguese pronunciation (IPA + tips):

apito → /ɐˈpitu/
- a = reduced, like the a in “about”
- stress on PI: a-PI-to
árbitro → /ˈaɾβitɾu/
- ár = stressed “a”, like “ah” (but shorter)
- r between vowels is a tapped ɾ (like the Spanish single r)
- b is slightly softened, can sound close to v in fast speech
- final o is often close to u in European Portuguese
- syllables: ÁR-bi-tro

Very roughly:

apito ≈ “uh-PEE-too”
árbitro ≈ “AR-bee-troo” (with a quick tapped r)