Si arribes aviat, et guardaré una butaca a la primera fila.

In Catalan, after si (if), it is normal to use the present tense when talking about a real future possibility:

• Si arribes aviat, et guardaré...
• literally: If you arrive early, I will save you...

This works much like Spanish, and differently from English, which also uses present after if: If you arrive early, I’ll save you a seat.

So arribes is present, while guardaré is future.

Arribes is the 2nd person singular present of arribar (to arrive).

• jo arribo = I arrive
• tu arribes = you arrive
• ell/ella arriba = he/she arrives

So si arribes aviat means if you arrive early.

Et is the object pronoun meaning to you or for you, depending on the context.

In this sentence:

• et guardaré una butaca

it means something like:

• I’ll save you a seat
• more literally: I’ll keep a seat for you

Catalan often uses a weak pronoun like this where English just uses you after the verb.

Some related forms are:

• em = me
• et = you
• li = him/her
• ens = us
• us = you (plural)
• els = them

Normally guardar means to keep, to save, to put away, or to guard, depending on context.

Here, guardar una butaca means:

• to save a seat
• to keep a seat free

So et guardaré una butaca means I’ll save a seat for you.

Guardaré is the 1st person singular future of guardar:

• jo guardaré = I will save / I will keep

The ending -é marks the future for jo in many verbs.

For example:

• arribaré = I will arrive
• parlaré = I will speak
• guardaré = I will save

The accent in guardaré shows the stress: guar-da-ré.

In Catalan, many future forms of jo end in -é and carry a written accent:

• faré = I will do
• aniré = I will go
• guardaré = I will save

This accent is important in spelling and pronunciation.

Aviat usually means early or soon, depending on context.

In this sentence, arribes aviat means you arrive early.

Compare:

• arribar aviat = to arrive early
• fins aviat = see you soon

So aviat is a flexible word, and context tells you whether it means early or soon.

Butaca usually means an armchair or a seat, especially an individual seat in places like a theater, cinema, or auditorium.

Seient is a more general word for seat.

So in this sentence, una butaca suggests a proper individual seat, probably in a venue with rows.

That is why it sounds natural with:

• a la primera fila = in the first row

A la primera fila means in the first row.

Breakdown:

• a = in / at / to
• la = the
• primera = first
• fila = row

Catalan often uses a in places where English uses in or at.

So:

• a la primera fila = in the first row
• literally closer to: at the first row

This is just normal Catalan usage.

Because fila (row) is a feminine noun in Catalan.

So the adjective must agree with it:

• la primera fila = the first row

Compare:

• el primer seient = the first seat
• la primera fila = the first row

Primer is masculine, primera is feminine.

More or less, yes:

• Si = if
• arribes = you arrive
• aviat = early
• et = to/for you
• guardaré = I will save
• una butaca = a seat
• a la primera fila = in the first row

A very literal version would be:

• If you arrive early, I will save for you a seat in the first row.

But natural English is:

• If you get here early, I’ll save you a seat in the front row / first row.

Yes. Very often la primera fila corresponds naturally to the front row in English.

So the sentence could be understood as:

• If you arrive early, I’ll save you a seat in the front row.

English often prefers front row in this kind of context, even though Catalan says first row.

The given word order is completely natural:

• Si arribes aviat, et guardaré una butaca a la primera fila.

Catalan does allow some flexibility, but not every change sounds equally natural.

For example, you could also say:

• Si arribes aviat, guardaré una butaca per a tu a la primera fila.

That version uses per a tu instead of the pronoun et, but the original sentence is smoother and more idiomatic.

So the original order is a very good model to learn.