Aquesta cançó m'agrada molt, i la vull compartir amb tu.

Because agradar works differently from English to like.

In Catalan, the thing that is pleasing is the grammatical subject, and the person who experiences the liking is shown with an indirect object pronoun.

So:

• Aquesta cançó m'agrada = literally, This song pleases me
• m' = to me
• agrada = pleases / is pleasing

That is why Catalan uses m'agrada instead of a direct equivalent of I like.

m' is the shortened form of em, the unstressed pronoun meaning to me.

Before a vowel or silent h, em usually becomes m':

• em gusta → not Catalan
• em sembla
• m'agrada
• m'interessa

So in this sentence:

• m'agrada = it pleases me / I like it

The verb agrees with the thing being liked.

Here, the subject is Aquesta cançó, which is singular, so the verb is singular:

• Aquesta cançó m'agrada = singular
• Aquestes cançons m'agraden = plural

So if you change song to songs, the verb also changes:

• Aquestes cançons m'agraden molt

Aquesta means this and it is the feminine singular form.

It has to match the noun cançó, which is feminine singular.

Common forms are:

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

So:

• aquest llibre = this book
• aquesta cançó = this song

The ç is called ce trencada. It represents an s sound before a, o, u.

So cançó is pronounced roughly like kan-SO.

A few useful points:

• ç sounds like s
• the final ó has a stressed o sound
• the written accent shows where the stress goes

So cançó is stressed on the last syllable.

Molt means a lot / very much here. It intensifies m'agrada.

So:

• m'agrada = I like it
• m'agrada molt = I like it a lot / very much

With agradar, Catalan commonly uses molt where English often uses a lot or very much.

La is a direct object pronoun meaning it, referring back to aquesta cançó.

Since cançó is feminine singular, the pronoun is la.

So:

• la vull compartir = I want to share it

This avoids repeating aquesta cançó.

Compare:

• Vull compartir aquesta cançó = I want to share this song
• La vull compartir = I want to share it

In Catalan, unstressed object pronouns usually come before the conjugated verb.

So:

• la vull compartir not
• vull la compartir

This is normal Catalan word order.

The structure is:

• la = it
• vull = I want
• compartir = to share

So literally: it I-want to-share.

Vull is the first person singular form of voler (to want).

So:

• vull = I want

A few forms of voler are:

• jo vull = I want
• tu vols = you want
• ell/ella vol = he/she wants
• nosaltres volem = we want

In the sentence, la vull compartir means I want to share it.

In this sentence, amb tu means with you, and it emphasizes sharing something together or with that person.

So:

• compartir amb tu = to share with you

Catalan can sometimes use other structures depending on the meaning, but compartir amb tu is very natural here.

Also note:

• amb = with
• tu = you (informal singular)

So amb tu is simply the prepositional phrase with you.

Yes. Catalan usually omits subject pronouns when they are not needed, because the verb form already shows the person.

So instead of saying:

• Jo la vull compartir amb tu

it is very normal to say:

• La vull compartir amb tu

The same applies earlier in the sentence:

• Aquesta cançó m'agrada molt

There is no need to say jo unless you want emphasis or contrast.

The comma separates two related clauses:

• Aquesta cançó m'agrada molt
• i la vull compartir amb tu

In short everyday writing, some people might omit the comma, but using it here is perfectly natural because the sentence contains two full parts and the pause sounds clear.

So the comma is about punctuation and rhythm, not about a special grammar rule unique to this sentence.