Aquestes botes negres combinen bé amb l'abric gris.

Because botes is a feminine plural noun, and the demonstrative has to agree with it.

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

So:

• aquesta bota = this boot
• aquestes botes = these boots

In Catalan, nouns have grammatical gender, and bota is a feminine noun. The plural is botes.

A good clue is the article:

• la bota = the boot
• les botes = the boots

Since it is feminine, related words also become feminine:

• aquestes botes
• botes negres

Because the subject is Aquestes botes negres, which is plural.

The verb combinar must agree with the subject:

• La bota combina = The boot goes well / matches
• Les botes combinen = The boots go well / match

So combinen is the third-person plural form.

Combinar amb means to go well with, to match, or to combine with.

In this sentence:

• combinen = go well / match
• bé = well
• amb = with

So the pattern is:

[thing] + combina/combinen bé amb + [other thing]

Examples:

• La camisa combina bé amb els pantalons.
• Aquestes sabates combinen bé amb el vestit.

The accent shows both pronunciation and meaning.

• bé = well
• be is not the same word in normal modern usage

So in combinen bé amb, bé is the adverb well.

You will see bé very often in Catalan:

• Està bé. = It’s fine.
• Parla bé català. = He/She speaks Catalan well.

In Catalan, adjectives often come after the noun, especially when they describe basic qualities like color.

So:

• botes negres = black boots
• abric gris = grey coat

This is more natural in Catalan than putting the adjective before the noun.

Some Catalan adjectives have the same plural form for both genders.

With negre:

• masculine singular: negre
• feminine singular: negra
• masculine plural: negres
• feminine plural: negres

So both of these are correct:

• pantalons negres = black trousers
• botes negres = black boots

Because gris is one of those adjectives that often has the same form for masculine and feminine singular.

So you can have:

• abric gris = grey coat
• camisa grisa is also possible in standard Catalan, but color adjectives like this can be a bit variable depending on usage and dialect

In this sentence, gris is the correct singular form agreeing with abric.

Its plural is usually grisos / grises when clearly inflected:

• abrics grisos

Because abric begins with a vowel, and the singular article el is apostrophized before a vowel sound.

So:

• el + abric becomes l'abric

This is very common in Catalan:

• l'home
• l'escola
• l'abric

It works similarly with la before many vowels too:

• l'amiga

Amb means with, and it is followed here by l'abric = the coat.

So:

• amb = with
• l'abric = the coat

Together:

• amb l'abric = with the coat

There is no special contraction between amb and the article here; the apostrophe belongs to l'abric, not to amb.

Yes. That is also natural Catalan.

Both are common:

• combinen bé amb = go well with / match well with
• van bé amb = go well with

Combinar sounds a bit more directly about visual matching or stylistic coordination, while anar bé amb can be slightly broader and very idiomatic.

Yes, in this sentence it works like English these.

It points to specific items that are near the speaker or relevant in the situation:

• aquestes botes = these boots

Catalan has a demonstrative system similar to English:

• aquest / aquesta / aquests / aquestes = this / these
• aquell / aquella / aquells / aquelles = that / those

So aquestes botes is the natural way to say these boots.

This is the normal and most natural order:

Aquestes botes negres combinen bé amb l'abric gris.

Catalan usually follows a standard subject + verb + complements order here:

• Aquestes botes negres = subject
• combinen = verb
• bé amb l'abric gris = complement

You could change the order for emphasis, but this version is the straightforward neutral one.