El botó d'aquest jersei és blanc, però la cremallera de la faldilla és negra.

Because de usually becomes d' before a vowel sound.

So:

• de + aquest → d'aquest
• de + aquella → d'aquella

This is very common in Catalan and makes the phrase flow more naturally.

Because two different things are happening:

• d'aquest jersei: de shortens to d' before the vowel at the start of aquest
• de la faldilla: la starts with a consonant, so there is no shortening

So:

• de + aquest → d'aquest
• de + la faldilla stays de la faldilla

Also, Catalan only makes the contractions del and dels with the definite articles el and els:

• de + el → del
• de + els → dels

But not with la, les, or with demonstratives like aquest.

Because jersei is a masculine singular noun.

The demonstrative has to agree with the noun:

• masculine singular: aquest
• feminine singular: aquesta
• masculine plural: aquests
• feminine plural: aquestes

So:

• aquest jersei
• aquesta faldilla

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

Here:

• el botó is masculine singular → blanc
• la cremallera is feminine singular → negra

So the adjective changes form to match the noun.

Compare:

• botó blanc
• cremallera negra
• faldilla negra
• jersei blanc

Because botó is masculine, not feminine.

Even though English adjectives do not change, Catalan adjectives often do:

• masculine: blanc
• feminine: blanca

So:

• el botó és blanc
• la camisa és blanca

És has a written accent to distinguish it from es.

• és = is (verb ser)
• es = a pronoun, as in reflexive forms

This is called a diacritical accent. It helps you tell two short words apart in writing.

In standard Catalan, però is always written with an accent.

It means but.

The accent shows the stressed vowel and is part of the normal spelling of the word. So you should learn it as però, not pero.

Because the stress falls on the last syllable: bo-TÓ.

In Catalan, words ending in a vowel are normally stressed on the second-to-last syllable unless an accent mark shows otherwise. Since botó is stressed on the last syllable, it needs the accent:

• bo-TÓ

So the accent tells you both how to pronounce it and how to stress it.

Not if you want to keep the same meaning.

• del jersei = of the sweater
• d'aquest jersei = of this sweater

So aquest adds the idea of this. If you replace it with del, you lose that meaning.

Also note the grammar:

• de + el jersei → del jersei
• but de + aquest jersei → d'aquest jersei

In this sentence, the second és is normally needed.

Catalan does not usually omit the verb to be the way some languages do. So:

• El botó d'aquest jersei és blanc, però la cremallera de la faldilla és negra.

sounds complete and natural.

Leaving out the second és would sound wrong or very unnatural in standard Catalan.

Because Catalan normally uses an article or another determiner with specific singular count nouns.

Here, each noun is specific:

• the button
• the zipper
• the skirt

So Catalan naturally uses:

• el botó
• la cremallera
• la faldilla

This is very normal. In many cases, Catalan uses articles more consistently than English learners may expect.

Yes. Jersei is a normal Catalan word for sweater / jumper.

It is a borrowed word, but it is fully established in Catalan spelling and usage. So you should write jersei, not the English spelling jersey.

A simple learner-friendly guide would be:

• jersei ≈ zher-SEI
• cremallera ≈ kreh-mah-YEH-rah or kreh-mah-LYEH-rah, depending on accent

A few helpful points:

• j in Catalan is usually like the s in measure or the zh sound
• ll can sound like a palatal l or sometimes more like y, depending on dialect
• the stressed syllable in jersei is the last one
• the stressed syllable in cremallera is lle

So even if pronunciation varies a bit by region, those approximations will help you say the words recognizably.