Quan arribin les convidades, la meva mare servirà l'amanida i jo serviré el pa.

Because this quan-clause refers to a future event that has not happened yet. In Catalan, that usually calls for the present subjunctive:

• Quan arribin les convidades... = when the guests arrive

If you say quan arriben, it more often sounds like a habitual or repeated situation:

• Quan arriben les convidades, sempre obrim una ampolla de vi.
  = When the guests arrive / Whenever the guests arrive, we always open a bottle of wine.

So in your sentence, arribin is the normal form.

Arribin is the present subjunctive, 3rd person plural, of arribar.

Here is the pattern:

• jo arribi
• tu arribis
• ell/ella arribi
• nosaltres arribem
• vosaltres arribeu
• ells/elles arribin

In the sentence, it matches les convidades, which is a plural subject, so arribin is the correct form.

Not in the same way.

• Quan arribin les convidades, ... = When the guests arrive, ...
• Quan arribaran les convidades? = When will the guests arrive?

So quan arribaran... is normally a direct question, not the kind of subordinate time clause used in your sentence.

In Catalan, possessives normally appear with the definite article:

• la meva mare = my mother
• el meu pare = my father
• la seva casa = his/her/their house

So the article is not extra emphasis here; it is the normal Catalan structure.

There are shorter forms like ma mare or mon pare, but la meva mare is the most neutral and widely useful form for learners.

Yes, Catalan often drops subject pronouns because the verb ending already shows the person:

• serviré already means I will serve

So you could say:

• ..., la meva mare servirà l'amanida i serviré el pa.

But jo is included here for contrast:

• la meva mare will serve the salad
• I will serve the bread

So jo is not required, but it sounds natural because it highlights the change of subject.

Convidades is the feminine plural form of convidada. So it means that the invited people are understood as female.

• les convidades = the female guests / the invited women
• els convidats = the male guests, or a mixed group in traditional grammar

So the sentence specifically uses the feminine plural. If the group were mixed or masculine, you would normally see els convidats.

Because amanida begins with a vowel, so la becomes l' before it:

• la + amanida → l'amanida

But pa begins with a consonant, so no apostrophe is needed:

• el pa

This is a very common Catalan spelling rule with articles:

• l'aigua
• l'escola
• l'amic
• la taula
• el llibre

Catalan uses the definite article much more often than English in cases like this. When talking about specific items that are part of the meal, the article is very natural:

• servir l'amanida
• servir el pa
• portar el vi

English often leaves the article out, but Catalan usually does not. So this sentence sounds fully normal.

They help show both pronunciation and verb form.

These are future forms of servir:

• serviré = I will serve
• servirà = he/she will serve

The accents show where the stress falls:

• serviRÉ
• serviRÀ

They are also part of the standard written endings of the future tense:

• -é for jo
• -às for tu
• -à for ell/ella/vostè
• -em for nosaltres
• -eu for vosaltres
• -an for ells/elles/vostès

Yes. Servir is the normal verb here.

• servir l'amanida = to serve the salad
• servir el pa = to serve the bread

Just be aware that servir can also mean to be useful / to work for a purpose, depending on context:

• Això serveix? = Is this useful? / Does this work?
• Serveix per tallar pa. = It is used for cutting bread.

In your sentence, the meaning is clearly to serve food.

Because Quan arribin les convidades is an introductory subordinate clause. In Catalan, it is very normal to separate that kind of clause from the main clause with a comma when it comes first:

• Quan arribin les convidades, la meva mare...

This is similar to English punctuation in sentences like:

• When the guests arrive, my mother will serve the salad...

So the comma is standard and helps mark the structure clearly.