Learn Hardcore Portuguese (Portugal): Ela não escondeu a surpresa quando viu todos os amigos na sala. - She did not hide her surprise when she saw all the friends in the room.

Ela não escondeu a surpresa… – fully explicit (emphasis on she).
Não escondeu a surpresa… – also correct; the subject is understood from escondeu (3rd person singular).

Using ela:

can add a bit of emphasis (contrasting her with others),
or make the subject crystal-clear in context.

Grammatically, both versions are fine.

Why is it não escondeu and not something like não escondiu?

Esconder (to hide) is a regular -er verb in the past simple (pretérito perfeito):

eu escondi
tu escondeste
ele/ela/você escondeu
nós escondemos
vocês/eles/elas esconderam

So the correct 3rd person singular is escondeu, not escondiu.
Não always comes right before the verb in simple tenses: não escondeu.

What tense is escondeu and why is it used here?

Escondeu is the pretérito perfeito simples (simple past), used for completed actions:

Ela não escondeu a surpresa = She did not hide her surprise (in that one situation), and the event is over.

If you said não escondia a surpresa, that would be imperfect and would suggest a habitual action:

Ela não escondia a surpresa = She didn’t (usually) hide her surprise / She was not one to hide her surprise.

Why do we say a surpresa and not just surpresa?

Portuguese uses the definite article a (the) more than English:

Ela não escondeu a surpresa
literally: She didn’t hide *the surprise
idiomatically: She didn’t hide **her surprise*.

The definite article is natural here because we’re talking about a specific, concrete feeling at that moment.

Without the article:

Ela não escondeu surpresa – sounds odd or stylistically marked; it might be used only in very particular, literary contexts.
In normal speech, you want a surpresa.

Is não escondeu a surpresa an idiom meaning “didn’t hide her feelings”?

Yes, it’s very close to the English idiom.

Literally:

esconder a surpresa = to hide the surprise

But in context, não escondeu a surpresa means:

she didn’t try to conceal the fact that she was surprised,
she showed her surprise on her face/in her reaction.

You can replace a surpresa with other feelings in the same structure:

não escondeu a alegria – didn’t hide her joy
não escondeu a tristeza – didn’t hide her sadness

What does viu mean exactly, and what verb form is it?

Viu is the 3rd person singular of ver (to see) in the same past tense:

Verb: ver
Tense: pretérito perfeito simples
Forms: eu vi, tu veste (EP), ele/ela/você viu, nós vimos, vocês/eles/elas viram

So quando viu = when she saw.
It matches the time frame of escondeu; both actions are completed in the past.

Why is it quando viu and not quando ela viu?

Because the subject is the same in both clauses (the “she” who didn’t hide, and the “she” who saw), Portuguese can drop ela in the second clause:

Ela não escondeu a surpresa quando viu… (most natural)
Ela não escondeu a surpresa quando ela viu… (also correct, but sounds a bit heavier)

You usually add ela again only if you need to avoid ambiguity or create contrast, e.g.:

Ela não escondeu a surpresa quando ela viu os amigos, mas ele fingiu que não os tinha visto.

What’s the difference between quando viu and quando via?

Both come from ver, but different past tenses:

quando viu – pretérito perfeito (completed, one moment)
- Ela não escondeu a surpresa quando viu todos os amigos na sala.
  One specific event: she saw them once, reacted once.
quando via – pretérito imperfeito (repeated/ongoing in the past)
- Ela não escondia a surpresa quando via os amigos.
  Means: Whenever she saw her friends, she wouldn’t hide her surprise.

So viu = a single, finished moment; via = habitual or ongoing past action.

Why is it todos os amigos and not todos amigos?

In Portuguese, when todos (all) is followed by a noun, you almost always need the definite article:

todos os amigos – all (the) friends
todas as pessoas – all (the) people

todos amigos is ungrammatical in standard Portuguese in this context.

If you want to be explicit that they are her friends, you can add a possessive:

todos os seus amigos – all her friends

But it’s very common to omit seus when it’s obvious whose friends they are from context.

Why amigos (masculine) if the group might include women?

Portuguese uses the masculine plural as the default for:

mixed-gender groups,
or groups whose gender composition is unknown or irrelevant.

So:

amigos can mean “male friends” or “male + female friends (mixed group)”.
amigas is specifically “female friends only”.

Thus todos os amigos is the normal generic way to say all (her) friends.

What is na in na sala? Why not just em a sala?

Na is a contraction:

em (in) + a (the, feminine singular) → na

So:

na sala = em a sala = in the room / in the living room.

Other examples:

no carro = em + o carro (in the car)
nas casas = em + as casas (in the houses)

You almost always use the contracted form (na, no, nas, nos) in speech and writing.
Em a sala is grammatically analyzable but not used in practice.

Does sala mean any room, or specifically a living room?

Sala by itself, in everyday speech, often suggests:

sala de estar – living room / lounge

But it can also be part of compound expressions:

sala de aula – classroom
sala de jantar – dining room
sala de reuniões – meeting room

In your sentence, na sala probably means in the (living) room where the friends were gathered, but context decides the exact type of room.

Is the word order fixed, or could we say something like Quando viu todos os amigos na sala, ela não escondeu a surpresa?

Both orders are correct:

Ela não escondeu a surpresa quando viu todos os amigos na sala.
Quando viu todos os amigos na sala, ela não escondeu a surpresa.

Portuguese is flexible with placing the time clause (quando…) at the beginning or the end.
What does not change is:

não comes directly before the verb: não escondeu
object after the verb: escondeu a surpresa (you wouldn’t say escondeu a surpresa não in this neutral sentence).