Els pantalons de la botiga són menys cars que la jaqueta.

Because pantalons is very commonly used in Catalan as a plural noun for the garment, much like English pants or trousers. Since the subject is plural, the rest of the sentence also uses plural forms: són and cars.

A singular form pantaló does exist, but in everyday usage els pantalons is the normal choice for the clothing item as a whole.

Here, de la botiga means from the shop or the shop’s. It tells you which pants we are talking about: the ones from that shop.

So els pantalons de la botiga = the pants from the shop.

It does not usually mean in the shop. For location, Catalan normally uses a:

• a la botiga = in/at the shop

Because són is the 3rd person plural form of ser, and the subject is plural: els pantalons.

• els pantalons són = the pants are
• la jaqueta és = the jacket is

So the verb agrees with els pantalons, not with la jaqueta.

Because adjectives in Catalan agree with the noun they describe in gender and number.

Here, cars describes pantalons, which is masculine plural, so the adjective must also be masculine plural:

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

So:

• els pantalons són cars
• la jaqueta és cara

Menys ... que is the Catalan pattern for less ... than.

So:

• menys cars que = less expensive than

Other examples:

• menys alt que = less tall than
• menys interessant que = less interesting than

The matching pattern for more ... than is més ... que.

In this sentence, que means than.

So:

• menys cars que la jaqueta = less expensive than the jacket

This is very common in Catalan comparisons:

• més gran que = bigger than
• menys ràpid que = less fast than

Catalan que can mean different things in different contexts, but after a comparative like més or menys, it usually means than.

Because cars describes els pantalons, not la jaqueta.

In a comparison like A són menys cars que B, the adjective agrees with A — the thing being described. B is just the thing you compare it to.

So:

• Els pantalons són menys cars que la jaqueta.
• La jaqueta és menys cara que els pantalons.

Notice how the adjective changes depending on the subject:

• pantalons → cars
• jaqueta → cara

In this sentence, it most naturally refers to els pantalons, because it comes right after that noun phrase:

• Els pantalons de la botiga ...

So the usual reading is the pants from the shop.

If you wanted de la botiga to refer to la jaqueta, you would normally place it after la jaqueta:

• Els pantalons són menys cars que la jaqueta de la botiga.

Word order helps show what each phrase modifies.

Menys cars is perfectly correct and easy to understand. It means less expensive.

That said, in everyday speech many speakers might more naturally say:

• més barats que la jaqueta = cheaper than the jacket

So these are both fine:

• són menys cars que la jaqueta
• són més barats que la jaqueta

The meaning is basically the same.

Because ser is the normal verb for describing something as expensive or cheap in a neutral, general way.

So:

• Els pantalons són cars
• La jaqueta és cara

Using estar can sometimes happen in specific contexts, especially when talking about a temporary situation or current prices, but ser is the standard and safest choice for this kind of comparison.

Yes, la jaqueta normally means the jacket, so it refers to a specific jacket that is known from the context.

If you were speaking more generally about jackets, you would usually use a different form, for example:

• que una jaqueta = than a jacket
• que les jaquetes = than the jackets

So in your sentence, the comparison is with one particular jacket.