La guia parla d'un quadre més i explica per què l'artista va canviar molt.

Because parlar usually takes the preposition de when it means to talk about.

• parlar de + noun = to talk about + noun
• parla d'un quadre = she/he talks about a painting

The de becomes d' before un for smoother pronunciation:

• de + un → d'un

So:

• parla d'un quadre = talks about a painting
• not parla un quadre, which would sound ungrammatical here

Quadre usually means painting or picture in this kind of art context.

It can also mean frame or even other things in different contexts, but here, because the sentence mentions l'artista, the natural meaning is painting.

So:

• un quadre = a painting
• d'un quadre més = about one more painting / about another painting

Here més means more, but in this sentence it is best understood as another or one more.

So:

• un quadre més = one more painting
• very naturally in English: another painting

Catalan often uses noun + més in this way:

• un dia més = one more day
• una pregunta més = one more question

That is the normal Catalan word order in this kind of expression.

Catalan commonly says:

• un llibre més = one more book
• una cosa més = one more thing
• un quadre més = one more painting

English often puts the idea into another before the noun, but Catalan frequently uses més after the noun.

So although English might say another painting, Catalan naturally says un quadre més.

This is a very common learner question.

In Catalan:

• per què = why / for what reason
• perquè = because

So in your sentence:

• explica per què... = explains why...

Compare:

• No sé per què va marxar. = I don’t know why he left.
• Va marxar perquè estava cansat. = He left because he was tired.

A useful shortcut:

• if it means why? → per què
• if it means because → perquè

Va canviar is the periphrastic past, one of the main ways Catalan talks about completed past actions.

Structure:

• va / vaig / vam / etc. + infinitive

So:

• va canviar = changed
• literally it looks like goes to change, but it does not mean future here

Examples:

• va parlar = spoke
• va arribar = arrived
• va canviar = changed

This tense is extremely common in spoken and written Catalan.

Because in Catalan, anar + infinitive can function as a past tense form, especially in the 3rd person singular:

• va canviar = changed

This is different from Spanish, where va a cambiar means is going to change. A native English speaker may expect a future meaning because of go + infinitive, but Catalan works differently here.

To express a true future, Catalan would normally use a future form such as:

• canviarà = will change

So:

• va canviar = changed
• canviarà = will change

Molt here means a lot or greatly.

So:

• va canviar molt = changed a lot

It comes after the verb phrase, which is normal in Catalan:

• ha treballat molt = has worked a lot
• va estudiar molt = studied a lot
• va canviar molt = changed a lot

Because the definite article el is reduced before a vowel.

• el artista → l'artista

This is called elision.

Other examples:

• l'home = the man
• l'escola = the school
• l'artista = the artist

So l'artista simply means the artist.

It can mean either, depending on context.

• la guia can be the female guide
• la guia can also be the guidebook

Without more context, both are possible. If the sentence is about a museum visit, la guia might mean the guide. If it is about a book or audio guide, it could mean the guidebook/guide.

Grammatically, both are fine.

Catalan often omits que in places where English might also sometimes omit that.

Here:

• explica per què l'artista va canviar molt

means:

• explains why the artist changed a lot

You do not need an extra que before per què.

The structure is:

• explica = explains
• per què... = why...

So the whole second part is simply explains why the artist changed a lot.

In this sentence, parla is most naturally she/he talks or it talks, because the subject is La guia.

• La guia parla... = The guide talks...

Catalan verbs often do not need subject pronouns, so the subject is understood from the noun already present.

Here the structure is:

• La guia = subject
• parla = talks
• explica = explains

So both verbs refer back to La guia.

Because Catalan, like other Romance languages, often avoids repeating the subject when it is already clear.

So:

• La guia parla d'un quadre més i explica...

means:

• The guide talks about another painting and explains...

The subject La guia applies to both parla and explica.

Repeating it would usually be unnecessary unless you wanted emphasis or contrast.