Quan tots els convidats siguin aquí, felicitarem l'oncle i li donarem el regal.

Because Catalan normally uses the present subjunctive after quan when the clause refers to a future situation.

So:

• Quan tots els convidats siguin aquí... = when all the guests are here / have arrived
• not quan tots els convidats seran aquí in standard Catalan

This is a very common pattern:

• Quan arribi, parlarem. = When he arrives, we’ll talk.
• Quan ho sàpiguen, t’avisaran. = When they find out, they’ll let you know.

If quan refers to something habitual or already factual, Catalan uses the indicative instead:

• Quan tots els convidats són aquí, comencem. = When all the guests are here, we start.

So in your sentence, siguin appears because the guests are not here yet at the moment of speaking.

Siguin is the 3rd person plural present subjunctive of ser.

Here is the present subjunctive of ser:

• jo sigui
• tu siguis
• ell/ella sigui
• nosaltres siguem
• vosaltres sigueu
• ells/elles siguin

It matches tots els convidats, which is plural, so siguin is the correct form.

This is a very common learner question, because English uses just to be, while Catalan has ser and estar.

In standard Catalan, ser is very normal with expressions of location like aquí, especially when the idea is simply be present / be there:

• Ja són aquí. = They’re here already.
• Quan siguin aquí... = When they’re here...

You may also hear estar in many varieties, especially in everyday speech, but ser is perfectly natural and standard in this sentence.

So the safest takeaway is:

• ser aquí = very common and correct for be here
• estar also exists, but do not assume Catalan uses it exactly the way Spanish or English would

Because tots els + noun means all the + noun.

So:

• tots els convidats = all the guests
• tots els llibres = all the books
• totes les cases = all the houses

The word tots agrees with a masculine plural noun:

• tot = all / whole (masculine singular)
• tota = whole (feminine singular)
• tots = all (masculine plural)
• totes = all (feminine plural)

Since convidats is masculine plural, tots is the right form.

Because the definite article el becomes l' before a vowel sound.

So:

• el oncle → l'oncle
• el amic → l'amic
• el avi → l'avi

This is just the normal Catalan elision of the article.

Because the two verbs take different kinds of objects.

1. felicitar

Felicitar takes a direct object:

• felicitar algú = to congratulate someone

So:

• felicitarem l'oncle

2. donar

Donar usually has:

• a direct object = the thing given
• an indirect object = the person receiving it

So in:

• li donarem el regal

the parts are:

• el regal = direct object
• li = indirect object, meaning to him

You could also say the full version:

• donarem el regal a l'oncle

and li is replacing a l'oncle.

Because l'oncle belongs to the first verb, felicitarem, not automatically to the second one.

The sentence is really:

• felicitarem l'oncle
  and
• li donarem el regal

With the second verb, Catalan still needs to mark the recipient:

• li = to him

Without li, the second part would feel incomplete, because donar needs someone to receive the gift.

So Catalan does not rely on English-style implied repetition here; it normally uses the clitic pronoun.

Because felicitar is normally a direct-object verb in Catalan.

So you say:

• felicitar l'oncle
• felicitar la mare
• felicitar els amics

not normally felicitar a l'oncle in standard Catalan.

This is important because learners often expect a preposition before a person, but Catalan does not use it here the way Spanish often does.

By contrast, with donar, the recipient is introduced by a or replaced by li:

• donar el regal a l'oncle
• li donar el regal

Li is an indirect object clitic pronoun meaning:

• to him
• to her
• to you (formal singular)

In this sentence, it means to him, referring to l'oncle.

Examples:

• Li escric una carta. = I’m writing him/her a letter.
• Li diré la veritat. = I’ll tell him/her the truth.
• Li donarem el regal. = We’ll give him the gift.

Because Catalan often uses the definite article when referring to a specific, identifiable thing.

Here, el regal means the gift, presumably a particular gift that both speaker and listener know about.

Compare:

• donarem un regal = we’ll give a gift
• donarem el regal = we’ll give the gift

So the article tells you this is a specific present, not just any present.

Because the actions will happen later, after the guests are here.

The endings are:

• felicitarem = we will congratulate
• donarem = we will give

The -em ending here is the 1st person plural future ending for -ar verbs:

• felicitar → felicitarem
• donar → donarem

So both verbs mean we will ...

Yes. You can also put the quan clause at the end:

• Felicitarem l'oncle i li donarem el regal quan tots els convidats siguin aquí.

That means the same thing.

Putting the quan clause first often gives it a bit more emphasis as the condition or timing for the main action.

Because the sentence begins with a subordinate time clause:

• Quan tots els convidats siguin aquí, ...

In Catalan, when a clause like this comes first, it is very common to separate it with a comma before the main clause.

So the comma helps show the structure:

• When all the guests are here,
  we’ll congratulate uncle and give him the gift.

If the quan clause comes at the end, the comma is often omitted:

• Felicitarem l'oncle i li donarem el regal quan tots els convidats siguin aquí.

No, not in this sentence.

Els would mean to them or them, depending on the structure, but here the recipient is singular: l'oncle.

So you need:

• li = to him

If the recipient were plural, then you would use els:

• Donarem el regal als oncles.
• Els donarem el regal. = We’ll give them the gift.

So:

• li = to him / her
• els = to them

It can do both, depending on context.

Felicitar commonly means:

• to congratulate
• to wish someone a happy birthday / happy nameday / congratulations for an occasion

So felicitarem l'oncle could fit many situations: a birthday, a celebration, good news, and so on.

The exact nuance depends on context, but grammatically it works the same way: felicitar algú.