El jersei gris combina bé amb aquests texans blaus.

Because the subject is El jersei gris. The phrase amb aquests texans blaus is just the complement.

So the verb agrees with jersei, not with texans.

Compare:

• El jersei gris combina bé amb aquests texans blaus.
• Aquests texans blaus combinen bé amb el jersei gris.

In the second sentence, the subject is aquests texans blaus, so the verb becomes combinen.

Here combinar means to match or to go well with, especially for clothes, colors, or style.

The pattern is:

combinar bé amb + noun

So:

• combina = matches / goes well
• bé = well
• amb = with

You will often hear this with clothes:

• Aquesta camisa combina amb la faldilla.
• El negre combina bé amb gairebé tot.

Because bé is the normal adverb meaning well, and here it modifies the verb combina.

• combina bé = matches well

Ben is usually used before adjectives, adverbs, or participles as an intensifier, often meaning something like very or properly/well in a different structure.

For example:

• ben blau = very blue
• ben fet = well done / properly done

So in this sentence, bé is the correct form.

In Catalan, descriptive adjectives like colors usually come after the noun.

So:

• jersei gris = grey sweater
• texans blaus = blue jeans

This is more normal than putting the color before the noun. English speakers often want to copy English word order, but Catalan usually does not do that with color adjectives.

Aquests means these and agrees with texans.

Since texans is:

• masculine
• plural

the demonstrative must also be:

• masculine
• plural

The forms are:

• aquest = this (masculine singular)
• aquesta = this (feminine singular)
• aquests = these (masculine plural)
• aquestes = these (feminine plural)

Because aquests already works as the determiner, just like these in English.

So Catalan says:

• aquests texans

not:

• els aquests texans

In standard Catalan, you normally do not use an article together with a demonstrative in this kind of phrase.

Because blaus agrees with texans.

Texans is masculine plural, so the adjective must also be masculine plural.

The forms of blau are:

• blau = masculine singular
• blava = feminine singular
• blaus = masculine plural
• blaves = feminine plural

So:

• texans blaus
• camisa blava
• ulls blaus

Yes, gris also agrees with the noun, but here it is masculine singular because it describes jersei.

Its forms are:

• gris = masculine singular
• grisa = feminine singular
• grisos = masculine plural
• grises = feminine plural

So in this sentence:

• el jersei gris is correct

But you could also have:

• la samarreta grisa
• els jerseis grisos

In everyday Catalan, texans is a very common word for jeans.

You may also hear:

• pantalons texans = jeans / denim trousers

Like in English, the clothing item is very often used in the plural:

• uns texans
• aquests texans

So even though the word may look surprising at first, it is completely normal Catalan.

A rough guide:

• jersei ≈ zher-SEI
• gris ≈ grees
• aquests ≈ uh-KESTS or a-KESTS, depending on accent
• texans ≈ te-SHANS
• blaus ≈ BLOWS, but as one syllable

A few useful notes:

• The j in jersei is not like English j in jeans. It is usually more like the sound in measure.
• The x in texans sounds like sh, not ks.
• The stress in texans is on the second syllable.

Pronunciation varies somewhat by dialect, but these approximations are a good starting point.

Yes. Va bé amb is very natural and common.

So you could say:

• El jersei gris va bé amb aquests texans blaus.

The difference is small:

• combina bé amb sounds a bit more specifically about matching stylistically or visually
• va bé amb is a bit broader and very everyday

Both are correct in this context.

Yes, but the original order is the most neutral and natural.

Normal:

• El jersei gris combina bé amb aquests texans blaus.

For emphasis, you could move the prepositional phrase:

• Amb aquests texans blaus, el jersei gris combina bé.

That sounds more marked, because you are highlighting with these blue jeans first. For a learner, the original version is the safest pattern to use.