Jo no ho havia fet mai, però decorar una festa amb flors i globus m'ha agradat.

Yes, Catalan usually does drop subject pronouns because the verb ending already shows the person.

So (Jo) no ho havia fet mai is grammatical both with and without Jo.

Including Jo can:

• add emphasis,
• make the contrast clearer,
• sound a bit more personal.

So here Jo is not required, but it helps highlight I had never done it before.

Ho is a neuter direct object pronoun. It often means it, but more specifically it refers to:

• an action,
• an idea,
• something just mentioned or understood from context.

In this sentence, ho refers to the activity of decorar una festa amb flors i globus.

So no ho havia fet mai means I had never done that before.

In Catalan, mai by itself often means ever in negative structures, and with no it gives the meaning never.

So:

• No ho havia fet mai = I had never done it
• literally something like I had not done it ever

This is very common in Catalan: negative meaning is often built with no + mai.

Havia fet is the past perfect (pluperfect) of fer.

It is used for something that happened before another past moment.

Here the idea is:

• in the past, there was a moment when the speaker decorated a party and liked it,
• before that past moment, they had never done it.

So:

• no ho havia fet mai = I had never done it before
• m'ha agradat = I liked it / I have liked it

Catalan sometimes mixes a pluperfect clause with a present perfect clause in a natural way when talking about a new experience and the reaction to it.

Fet is the past participle of fer.

The verb fer works like this:

• infinitive: fer = to do / to make
• past participle: fet = done / made

The tense havia fet is built with:

• havia = imperfect of haver
• fet = past participle

So havia fet literally means had done.

In Catalan, an infinitive can act like a noun or a whole activity.

So decorar una festa amb flors i globus means:

• decorating a party with flowers and balloons

This whole infinitive phrase is the thing that pleased the speaker.

Catalan does this very naturally:

• Llegir m’agrada = I like reading
• Cuinar m’encanta = I love cooking
• Decorar una festa... m’ha agradat = I liked decorating a party...

This is one of the most important things to learn about agradar.

Agradar does not work like English to like.
It works more like to please.

So:

• m’ha agradat literally means it has pleased me
• natural English: I liked it

The structure is:

• thing liked = subject of the verb
• person who likes it = indirect object pronoun

Here:

• decorar una festa amb flors i globus = the thing that pleased
• m’ = to me
• ha agradat = has pleased

So he agradat would mean I have pleased or I was pleasing, which is not the intended meaning.

Because the grammatical subject is the whole infinitive phrase:

decorar una festa amb flors i globus

That whole activity is treated as one thing, so the verb is singular:

• m’ha agradat

Compare:

• Decorar una festa m’ha agradat = singular
• Les flors m’han agradat = plural, because les flors is plural

So the verb agrees with the thing doing the pleasing, not with the person who feels the liking.

M'ha is a contraction of:

• em ha

Catalan often shortens weak pronouns before a vowel.

So:

• em ha agradat → m’ha agradat

This is completely normal and very common.

Other examples:

• em agrada → m’agrada
• et ha dit → t’ha dit
• es havia aixecat → s’havia aixecat

Because amb flors i globus here means with flowers and balloons in a general, descriptive way.

Catalan often omits the article when talking about materials, objects, or means used in a general sense.

So:

• amb flors i globus = with flowers and balloons not necessarily
• with the flowers and the balloons

If you said amb les flors i els globus, it would sound more like you mean specific flowers and specific balloons already known in the context.

Yes. Però means but, and it introduces a contrast between the two ideas:

• Jo no ho havia fet mai = this was new for me
• ...m'ha agradat = and I liked it

So the contrast is:

• it was unfamiliar,
• yet the speaker enjoyed it.

That is exactly the kind of contrast però is used for.

Not naturally in this version.

If you say:

• Jo no havia fet mai...

the listener may expect you to continue directly with what you had never done.

But in:

• Jo no ho havia fet mai

the pronoun ho neatly stands in for that / it, referring to the activity understood from the sentence.

So ho helps make the first clause complete and natural:

• I had never done that before

Very often, yes, especially in simple negative sentences:

• No ho havia fet mai
• No hi vaig anar mai
• No l’he vist mai

Placing mai near the end is very common and natural.

Its position can vary a little in more complex sentences, but for a learner, the safest pattern is:

no + pronoun(s) + verb + mai

Yes, you could say No ho havia fet abans, but it is slightly different.

• No ho havia fet mai = I had never done it before / ever
• No ho havia fet abans = I hadn’t done it before

Mai emphasizes never at any time up to that point.
Abans simply means before.

In this sentence, mai is stronger and more idiomatic because the speaker is talking about a first-time experience.