Les sabatilles blanques de la meva filla són més còmodes que les botes.

Les is the feminine plural definite article, so it means the for feminine plural nouns.

It appears with:

• les sabatilles = the shoes/trainers/slippers
• les botes = the boots

Both sabatilles and botes are feminine plural nouns, so they both take les.

In Catalan, adjectives usually come after the noun. That is especially true for color adjectives.

So the normal order is:

• sabatilles blanques = white shoes

Putting the adjective before the noun is much less common and usually sounds literary, marked, or changes the nuance.

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

The base forms are:

• blanc = masculine singular
• blanca = feminine singular
• blancs = masculine plural
• blanques = feminine plural

Since sabatilles is feminine plural, the adjective must also be feminine plural: blanques.

Also, the spelling changes from c to qu before e:

• blanc → blanques

This keeps the hard k sound. If it were written with just c before e, it would sound different.

In standard Catalan, possessives are very often used with the definite article.

So Catalan normally says:

• la meva filla = my daughter
• el meu pare = my father
• les meves sabates = my shoes

For a learner, article + possessive + noun is the safest and most natural pattern to use.

Because the possessive must agree with the noun it goes with.

Here it goes with filla, which is:

• feminine
• singular

So you need meva.

Compare:

• el meu fill = my son
• la meva filla = my daughter
• els meus fills = my sons / my children
• les meves filles = my daughters

Catalan very commonly expresses possession with de + the possessor, especially when the possessor is a full noun phrase.

So:

• les sabatilles de la meva filla literally means the shoes of my daughter, but in natural English we say my daughter’s shoes

This is a very normal Catalan structure. When the possessor is a full noun like la meva filla, de is the standard way to link it.

Here Catalan uses ser because the sentence is describing a general quality of the shoes: they are more comfortable than the boots.

So:

• són més còmodes = they are more comfortable

As a rule of thumb:

• ser is used for characteristics, identity, classification, and general qualities
• estar is more often used for states or conditions

With clothes or shoes, ser còmodes is the normal way to say they are comfortable items.

This is the normal Catalan comparative structure:

• més + adjective + que = more + adjective + than

So:

• més còmodes que les botes = more comfortable than the boots

Other useful patterns are:

• menys + adjective + que = less ... than
• tan + adjective + com = as ... as

Also, més does not change form. It stays the same no matter the gender or number.

Because còmode is the adjective comfortable, and here it has to agree with sabatilles, which is feminine plural.

The forms are:

• còmode = masculine singular
• còmoda = feminine singular
• còmodes = plural feminine / also plural masculine in some contexts depending on the adjective pattern, but here it is feminine plural because of sabatilles

The accent in còmodes is part of the normal spelling and pronunciation of the word.

Yes, if you mean the boots.

After que, Catalan still keeps the article when the comparison is with a definite noun phrase:

• més còmodes que les botes

You cannot just drop it and say que botes here.

So the second les is not extra; it is simply the article belonging to botes.

By itself, les botes usually means the boots, so it refers to some specific boots already understood from the context.

Very often, the listener will assume they are the boots being compared with the shoes, possibly the daughter’s boots if that makes sense in context.

If you want to make it completely explicit, you could say:

• que les botes de la meva filla
• que les seves botes = than her boots

So the original sentence is fine, but context decides exactly which boots are meant.

Yes. que no pas is a common alternative after comparatives.

So both are correct:

• més còmodes que les botes
• més còmodes que no pas les botes

The version with no pas can sound a bit more emphatic or formal, but the basic meaning is the same.