No vull que l'oncle descobreixi la sorpresa abans de la festa.

Because Catalan usually uses voler que + subjunctive when the second action has a different subject.

In this sentence:

• No vull = I don’t want
• que l'oncle descobreixi... = that the uncle discover...

So the structure is literally closer to:

• I don’t want that my/the uncle discover...

But in natural English, we say:

• I don’t want my/the uncle to discover...

A useful comparison:

• No vull descobrir la sorpresa. = I don’t want to discover the surprise.
  
  • same subject: I don’t want to discover
• No vull que l'oncle descobreixi la sorpresa. = I don’t want the uncle to discover the surprise.
  
  • different subject: I don’t want the uncle to discover

Because after No vull que..., Catalan requires the subjunctive, not the indicative.

So:

• descobreix = indicative present, he/she discovers
• descobreixi = present subjunctive, used after expressions of desire, doubt, emotion, etc.

Since voler que expresses a wish or desire, the verb in the subordinate clause goes into the subjunctive:

• Vull que vingui. = I want him/her to come.
• No vull que marxin. = I don’t want them to leave.
• No vull que l'oncle descobreixi la sorpresa.

This is one of the most important patterns in Catalan grammar.

L' is the definite article that appears before a vowel sound.

Here, oncle begins with a vowel, so el oncle becomes:

• l'oncle

This is called elision.

Compare:

• el noi = the boy
• l'amic = the friend
• l'oncle = the uncle

So l'oncle simply means the uncle.

Vull is the 1st person singular present form of voler, which means to want.

So:

• jo vull = I want
• tu vols = you want
• ell/ella vol = he/she wants
• nosaltres volem = we want
• vosaltres voleu = you all want
• ells/elles volen = they want

In the sentence:

• No vull = I don’t want

Catalan uses definite articles very naturally with specific nouns, often more regularly than English.

Here they all refer to specific things already understood in the context:

• l'oncle = the uncle
• la sorpresa = the surprise
• la festa = the party

English sometimes omits articles in places where Catalan keeps them, but in this sentence the articles are perfectly normal in both languages.

Also, with family members, Catalan often uses the article very naturally:

• la mare = mother / the mother
• l'avi = grandfather / the grandfather
• l'oncle = uncle / the uncle

Depending on context, l'oncle may sound like the uncle or simply uncle in a familiar family context.

Because abans de is used before a noun or an infinitive, while abans que is used before a full clause.

Here, la festa is a noun phrase, so you use:

• abans de la festa = before the party

Compare:

• abans de la festa = before the party
• abans de sortir = before leaving
• abans que comenci la festa = before the party starts

So:

• abans de + noun/infinitive
• abans que + conjugated verb

Because descobrir belongs to a group of Catalan -ir verbs that often insert -eix- in certain forms. These are commonly called incoative verbs.

So from descobrir you get forms like:

• descobreixo = I discover
• descobreixes = you discover
• descobreix = he/she discovers
• descobreixi = that he/she discover

This pattern is very common in Catalan. Other verbs behave similarly:

• servir → serveix, serveixi
• preferir → prefereix, prefereixi
• decidir → decideix, decideixi

So descobreixi may look complicated, but it follows a regular pattern for this verb type.

Yes, but the given order is the most neutral and natural.

Standard order here is:

• No vull
• que l'oncle descobreixi la sorpresa
• abans de la festa

That is very straightforward and idiomatic.

Catalan can change word order for emphasis, but it may sound less neutral. For example, moving abans de la festa earlier could emphasize the timing:

• No vull que l'oncle, abans de la festa, descobreixi la sorpresa.

That is possible, but the original sentence is the clearest everyday version.

Yes. It is a very typical Catalan way to express this idea.

Catalan often uses:

• [main verb of wanting] + que + subjunctive

where English often prefers:

• [main verb] + object + infinitive

So:

• No vull que l'oncle descobreixi la sorpresa.
• natural English: I don’t want the uncle to discover the surprise.

This is one of those places where Catalan and English organize the sentence differently, even though the meaning is the same.