El que més m'agrada d'aquest abric és que és negre.

El que is a very common Catalan structure meaning what or the thing that.

So El que més m'agrada means what I like most / the thing I like most.

Here, el is not referring to abric. It is part of the fixed expression el que.

Catalan agradar does not work like English to like. It works more like to be pleasing to.

So:

• m' = to me
• agrada = is pleasing

That is why m'agrada corresponds to I like it in English.

This is also why the verb changes depending on whether the thing liked is singular or plural:

• M'agrada = I like it
• M'agraden = I like them

They are the same pronoun in different forms.

• em is the full form
• before a vowel, it usually becomes m'

So:

• em sembla
• m'agrada

This apostrophe is normal Catalan spelling.

més means more or most.

In El que més m'agrada, it gives the idea of the thing I like most.

It is creating a superlative idea, not just a comparison.

Also, the accent matters:

• més = more / most
• mes = month

Yes. El que més m'agrada and El que m'agrada més are both possible.

Both mean essentially the same thing. The difference is mostly one of emphasis and rhythm:

• El que més m'agrada is a very common way to say what I like most
• El que m'agrada més is also natural, especially in speech

This is de + aquest abric.

Here de means something like about or of:

• El que més m'agrada d'aquest abric = the thing I like most about this coat

Because aquest begins with a vowel sound, de becomes d':

• de aquest → d'aquest

That apostrophe is required.

Because abric is a masculine singular noun.

So the demonstrative must match it:

• aquest abric = this coat
• aquesta jaqueta = this jacket

Catalan demonstratives agree in gender and number with the noun.

Because the adjective agrees with abric, which is masculine singular.

So:

• abric negre
• jaqueta negra

In que és negre, the understood subject is still the coat, so the adjective stays masculine singular.

The two instances of que are doing different jobs.

1. In el que, it is part of the structure meaning what / the thing that
2. In és que és negre, it introduces a whole clause: that it is black

So even though the same word appears twice, the grammar is different each time.

Because the sentence is not saying that the thing I like most is black. It is saying that the thing I like most is that it is black.

So the second part is a full clause, and Catalan introduces that clause with que:

• és que és negre

Without que, the structure would not express the same idea correctly.

Catalan often leaves subject pronouns unstated when they are understood from context.

In English, you must say it is black.
In Catalan, the subject can be omitted:

• és negre = it is black

Here, the listener understands that the subject is the coat.

The accent marks are important in Catalan spelling.

• és = is
• es = a pronoun, as in reflexive forms

and

• més = more / most
• mes = month

So the accents help distinguish words that would otherwise look the same.