Jo prefereixo els texans quan fa fred, però la meva amiga prefereix la samarreta quan és a casa.

That’s right: Catalan often leaves out subject pronouns because the verb ending already tells you who the subject is.

Here, prefereixo already means I prefer, so jo is not required. It is included for one of these reasons:

• emphasis
• contrast
• clarity

In this sentence, it helps set up a contrast with la meva amiga:

• Jo prefereixo... però la meva amiga prefereix...

Without jo, the sentence would still be perfectly natural:

• Prefereixo els texans quan fa fred...

So jo is optional, but useful for emphasis.

Those are different present-tense forms of the verb preferir.

In this sentence:

• prefereixo = I prefer
• prefereix = he/she prefers

Preferir belongs to a group of Catalan -ir verbs that often add -eix- in several present-tense forms. For example:

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

So this is not random spelling; it is part of the normal conjugation pattern of the verb.

Catalan uses articles more often than English does, especially with clothing and general preferences.

Here:

• els texans = jeans
• la samarreta = the T-shirt literally, but in context it can simply mean a T-shirt / the T-shirt kind of clothing

When talking about clothes in a general or habitual way, Catalan commonly uses the definite article:

• M’agraden els vestits = I like dresses
• Porta la jaqueta = He/She wears a jacket

So the article is very normal here and does not always map directly to English usage.

Yes. Els texans is a very common Catalan word for jeans.

A few useful points:

• It is normally used in the plural, like English jeans
• The singular is much less common in everyday use
• The word can also mean Texans in another context, but here the clothing meaning is obvious

So in this sentence, els texans clearly means the item of clothing.

In standard Catalan, possessives usually appear together with the definite article when they go before a noun:

• la meva amiga = my friend
• el meu germà = my brother
• les nostres coses = our things

This is different from English, where my, your, our, etc. usually appear without an article.

So la meva amiga is the normal Catalan structure.

There are some exceptions in certain expressions or forms of address, but for ordinary noun phrases, article + possessive is the rule.

Because Catalan often uses the verb fer in weather expressions.

So:

• fa fred = it is cold
• fa calor = it is hot
• fa vent = it is windy
• fa sol = it is sunny

Literally, fa fred is something like it makes cold, but you should learn it as a fixed weather expression meaning it’s cold.

This is one of those places where Catalan and English organize the idea differently.

Quan means when, and in sentences like this it often has the sense of whenever.

So:

• Jo prefereixo els texans quan fa fred
  = I prefer jeans when it’s cold / whenever it’s cold

This describes a habitual situation.

If you said si fa fred, that would mean if it’s cold, which sounds more conditional:

• Si fa fred, em poso l’abric = If it’s cold, I put on my coat

So quan fits well here because the sentence is talking about what the speaker generally prefers in that kind of situation.

Quan és a casa means when she is at home.

Two important things are happening here:

1. a casa is the normal Catalan expression for at home
2. It usually appears without an article

So Catalan says:

• Sóc a casa = I’m at home
• És a casa = He/She is at home

This is a fixed, very common expression. You do not normally say a la casa for at home in this sense.

Also, casa can be extended:

• a casa meva = at my home
• a casa seva = at his/her home
• a casa dels avis = at the grandparents’ house

Yes, many speakers do also say està a casa, and you will hear both.

In Catalan, the choice between ser and estar is not always the same as in Spanish or English. In expressions of location, especially common everyday ones like és a casa, ser can sound perfectly natural.

So:

• és a casa = normal
• està a casa = also common

The preference can vary somewhat by dialect, style, and nuance, but for a learner the key point is:

• és a casa is not a mistake
• it is a natural Catalan way to say is at home

Because Catalan usually leaves out subject pronouns when the subject is clear from context.

In this sentence:

• la meva amiga prefereix la samarreta quan és a casa

once la meva amiga has been introduced, the verb és clearly refers back to her. So Catalan does not need to say ella.

You could include ella for emphasis, but normally it is unnecessary.

This is very common in Catalan:

• La Maria canta quan està contenta
  = Maria sings when she is happy

There is no need to repeat she every time, because the verb and context already make the reference clear.