Prefiero el antiguo, porque el nuevo me parece demasiado complicado.

Because preferir is a stem-changing verb. In the present tense, the e in the stem changes to ie in most forms:

• yo prefiero
• tú prefieres
• él/ella prefiere
• nosotros preferimos
• vosotros preferís
• ellos prefieren

So prefiero means I prefer.

In Spanish, adjectives can often be used on their own when the noun is understood from context. Here, el antiguo means something like the old one / the former one, and el nuevo means the new one.

This is very common in Spanish. For example:

• Quiero el rojo = I want the red one
• Prefiero el pequeño = I prefer the small one

So the missing noun might be something like móvil, modelo, sistema, etc., but Spanish does not need to repeat it.

Both can relate to age, but they are not always interchangeable.

• antiguo often means old, ancient, or former/previous
• viejo usually means old in the sense of aged or not new

In this sentence, antiguo works well because it often sounds like the previous one or the older version, especially when contrasted with el nuevo.

So el antiguo here is likely closer to the old/previous one than the worn-out one.

Parecer often works like to seem or to seem to someone.

• Me parece complicado = It seems complicated to me
• Te parece complicado = It seems complicated to you
• Nos parece complicado = It seems complicated to us

The me shows whose opinion it is. So el nuevo me parece demasiado complicado literally means the new one seems too complicated to me.

In natural English, we usually say the new one seems too complicated to me or simply I find the new one too complicated.

Spanish word order is flexible, but this order is very natural.

A rough breakdown is:

• el nuevo = the subject, the new one
• me = to me
• parece = seems
• demasiado complicado = too complicated

So literally: the new one seems to me too complicated.

English usually prefers: The new one seems too complicated to me.

Here demasiado means too in the sense of more than is desirable.

• demasiado complicado = too complicated
• demasiado caro = too expensive
• demasiado tarde = too late

Be careful: demasiado can also mean too much or too many in other contexts, depending on how it is used.

The comma is optional in many cases like this, but it is quite natural when the speaker pauses before giving the reason.

• Prefiero el antiguo porque... = also correct
• Prefiero el antiguo, porque... = also correct

In modern Spanish, many writers would often omit the comma before porque in a short sentence like this, but seeing it is not unusual.

Because porque is the conjunction meaning because.

Compare:

• porque = because
• por qué = why
• porqué = the reason / the why
• porque (less common as a noun-like form in older structures) is not the one here

In this sentence, the speaker is giving a reason, so porque is correct.

Yes, grammatically you could, but the meaning may shift slightly.

• el antiguo often suggests the former one / the previous version
• el viejo can sound more like the old one in the sense of aged, old-fashioned, or even worn out

If you are comparing an old version with a new version, el antiguo is often the better choice.

Not really in this sentence. Here parecer means to seem.

• Me parece complicado = It seems complicated to me
• Parece fácil = It seems easy

Spanish parecer can sometimes overlap with English to look, but only in certain contexts:

• Parece cansado = He looks tired / He seems tired

In your sentence, seems is the safest translation.

Because it agrees with el nuevo, which stands for a masculine singular noun.

Even though the noun is omitted, the adjective still has to match the implied noun:

• el nuevo ... complicado = masculine singular
• la nueva ... complicada = feminine singular
• los nuevos ... complicados = masculine plural
• las nuevas ... complicadas = feminine plural

So complicado matches the hidden noun behind el nuevo.

Yes. A more direct version would be:

• Prefiero el antiguo, porque el nuevo es demasiado complicado.

That means I prefer the old one, because the new one is too complicated.

The version with me parece is slightly softer and more subjective. It sounds more like an opinion:

• es demasiado complicado = more direct, more definite
• me parece demasiado complicado = it seems too complicated to me, more personal and less absolute

Yes, it sounds natural in Spain.

It is a normal way to express a preference and give a reason. In everyday speech in Spain, someone might also say:

• Prefiero el antiguo, porque el nuevo me parece muy complicado.
• Me quedo con el antiguo; el nuevo me parece demasiado complicado.
• Yo prefiero el antiguo, porque el nuevo es demasiado complicado para mí.

Your original sentence is perfectly natural and idiomatic.