Jo prefereixo aquest; l'altre és massa car.

Yes. Catalan often drops subject pronouns because the verb ending already tells you who the subject is. So Prefereixo aquest is perfectly natural.

Adding jo usually gives emphasis, contrast, or clarity. In this sentence, Jo prefereixo aquest can feel a bit like I prefer this one.

Prefereixo is the 1st person singular present indicative of preferir: I prefer.

This verb follows the common Catalan -eix- pattern in many present-tense forms:

• jo prefereixo
• tu prefereixes
• ell/ella prefereix
• nosaltres preferim
• vosaltres preferiu
• ells/elles prefereixen

So it is not completely regular in the simplest sense, but it follows a very common pattern.

A useful approximation is pre-fe-REI-shu in many accents.

A few helpful points:

• the stress falls on -rei-
• the x in -eix- sounds like English sh
• the ending -o is an unstressed vowel, so it may sound weaker than English oh

Exact pronunciation varies by dialect, but the key thing is that -eix- gives you a sh sound.

Because aquest can work as a demonstrative pronoun, not just a demonstrative adjective.

So:

• aquest llibre = this book
• aquest = this one

In the sentence, the noun is understood from context. Catalan does this very naturally.

Aquest means this or this one and agrees with a specific noun, here a masculine singular one.

Això is neuter and is used for:

• ideas
• situations
• something unspecified
• this thing

So if you are choosing between two specific objects, aquest is the normal choice. Això would sound less specific and would not usually fit as well here.

Here l'altre means the other one. Catalan normally uses the definite article in this kind of structure when the noun is omitted but understood.

So:

• l'altre = the other one
• l'altra = the other one for a feminine noun

Using just altre here would usually not sound right in standard Catalan.

Because the article el becomes l' before a vowel.

So:

• el + altre becomes l'altre

This is standard elision in Catalan. The same thing happens in many other cases, such as:

• l'amic
• l'home
• l'altra

The accent distinguishes the verb és from es.

• és = is
• es = a pronoun, often reflexive or object-related

So in this sentence, és is the 3rd person singular of ser: is. The accent is required.

Massa means too, while molt means very.

So:

• massa car = too expensive
• molt car = very expensive

That is an important difference. Massa implies excess, not just a high degree.

Also, car agrees with the implied noun:

• masculine singular: car
• feminine singular: cara
• masculine plural: cars
• feminine plural: cares

For example:

• Jo prefereixo aquesta; l'altra és massa cara.

Yes. The semicolon is natural because it links two closely related clauses:

• Jo prefereixo aquest
• l'altre és massa car

A full stop would also be fine:

• Jo prefereixo aquest. L'altre és massa car.

A comma is sometimes seen in less formal writing, but the semicolon is a neat and clear choice here.