Encara no sé si compraré el jersei gris o el negre, però el blau no m'agrada.

Encara no means not yet.

So:

• No sé = I don’t know
• Encara no sé = I still don’t know / I don’t know yet

It adds the idea that the decision has not been made so far, but it may be made later.

Yes, si often means if, but after verbs like saber (to know), preguntar (to ask), or veure (to see), it can introduce an indirect yes/no question.

So:

• No sé si compraré... = I don’t know whether I’ll buy...

In English, whether is often the most natural translation here, even though Catalan uses si.

Compraré is the first person singular future of comprar:

• comprar = to buy
• compraré = I will buy

It is used because the speaker is talking about a future decision.

So No sé si compraré... means I don’t know whether I’ll buy...

This is normal after si when si means whether.
It is different from a conditional sentence like Si tinc diners, el compraré (If I have money, I’ll buy it).

Because Catalan, like English, can leave out a noun if it is already understood.

• el jersei gris = the grey sweater
• el negre = the black one

The noun jersei is omitted the second time because it would be repetitive. The same thing happens with el blau, which means the blue one.

Because negre and blau are acting like short forms of a noun phrase:

• el negre = the black one
• el blau = the blue one

Once jersei has already been mentioned, the color adjective can stand on its own, but it still needs the article el.

Compare:

• el jersei gris = the grey sweater
• el negre = the black one
• el blau = the blue one

In Catalan, color adjectives normally come after the noun.

So:

• el jersei gris
• la camisa blava
• els pantalons negres

This is one of the most common adjective patterns in Catalan. English learners often want to put the adjective before the noun, but with colors that is usually not the normal order.

The adjectives agree with jersei, which is masculine singular:

• el jersei gris
• el jersei negre
• el jersei blau

Because jersei is masculine singular, the colors also appear in the masculine singular form.

If the noun were feminine, the forms would change:

• la samarreta negra
• la samarreta blava

Catalan uses agradar differently from English to like.

A helpful literal idea is:

• m’agrada = it pleases me

So in:

• el blau no m’agrada

the thing that is liked or disliked (el blau) is the grammatical subject.

That is why Catalan does not structure it like English I don’t like the blue one. Instead, it is closer to:

• the blue one does not please me

Because the subject is singular:

• el blau = the blue one

Since that is singular, the verb is singular too:

• el blau no m’agrada

If the subject were plural, you would use agraden:

• els blaus no m’agraden = I don’t like the blue ones

So with agradar, the verb agrees with the thing liked, not with the person who likes it.

M’ is the shortened form of em, the pronoun meaning to me.

So:

• m’agrada = em agrada = it pleases me / I like it

The apostrophe is used because em becomes m’ before a vowel:

• m’agrada
• m’estima
• m’interessa

This is very common in Catalan.

Però means but.

It connects two contrasting ideas:

• the speaker is undecided between the grey and black sweater
• but the blue one is definitely not liked

So:

• ..., però el blau no m’agrada.
  = ..., but I don’t like the blue one.

It is a very common coordinating conjunction in Catalan.