Abans que arribi la meva amiga, reservaré una taula a la terrassa del restaurant.

Because abans que normally triggers the subjunctive in Catalan when the action has not happened yet.

In this sentence, your friend has not arrived yet, so Catalan uses:

• arribi = present subjunctive
• not arriba = present indicative

So Abans que arribi la meva amiga is the natural way to say Before my friend arrives.

Arribi is the present subjunctive, first or third person singular, of the verb arribar.

Here it means she arrives in a subordinate clause:

• que arribi = that she arrive / arrives

Even though it is called present subjunctive, it can refer to the future, depending on context. In this sentence, it clearly refers to a future event.

Reservaré is the simple future, first person singular, of reservar:

• reservaré = I will reserve

Catalan often uses the future tense just as English does here. It tells you the reservation will happen later.

You may also hear the present used for near-future meaning in Catalan in some contexts, but reservaré is very clear and fully standard.

Catalan often omits subject pronouns because the verb ending already shows who the subject is.

• reservaré already means I will reserve

So:

• Reservaré una taula = I will reserve a table
• Jo reservaré una taula = also correct, but more emphatic

You would add jo mainly for contrast or emphasis.

In standard Catalan, possessives usually go with the definite article:

• la meva amiga = my friend
• el meu cotxe = my car
• les nostres claus = our keys

So the article la is normally expected here.

Also, meva agrees with amiga, which is feminine singular:

• meu = masculine singular
• meva = feminine singular

That is why you get la meva amiga, not el meu amiga.

Una taula uses the indefinite article because it means a table, not a specific already-identified table.

• una taula = any table / a table
• la taula = the table, a specific one already known

Since the speaker is talking about making a reservation and not referring to one particular previously mentioned table, una taula is the natural choice.

Here a is the normal preposition for location with places like this:

• a la terrassa
• al restaurant
• a casa

So reservar una taula a la terrassa means to reserve a table on the terrace.

You may also encounter en in Catalan, but a la terrassa is the natural choice here. Also note that Catalan does not say en la in this sentence.

Del is a contraction of:

• de + el = del

So:

• la terrassa del restaurant literally = the terrace of the restaurant
• natural English = the restaurant’s terrace

Catalan commonly uses these contractions:

• del = de + el
• al = a + el
• pel = per + el

Yes. That is also correct.

Catalan allows some flexibility in word order in subordinate clauses:

• Abans que arribi la meva amiga
• Abans que la meva amiga arribi

Both are grammatical. The version with the verb before the subject often sounds very natural and fluid, especially when introducing new information. The version with the subject first can feel a bit more explicit.

Because the sentence begins with a subordinate clause:

• Abans que arribi la meva amiga, ...

When this kind of introductory clause comes first, Catalan normally separates it from the main clause with a comma.

So the structure is:

• subordinate clause: Abans que arribi la meva amiga
• main clause: reservaré una taula a la terrassa del restaurant

Not in this sentence.

Use abans de + infinitive when you have an infinitive construction, often with the same subject as the main clause:

• Abans d'entrar al restaurant, reservaré una taula.
• Before entering the restaurant, I’ll reserve a table.

But here the person who arrives is la meva amiga, not the speaker. Because there is a full clause with its own verb, Catalan uses:

• abans que arribi la meva amiga

So abans que is the right structure here.