Vull fer una sorpresa a la meva mare dissabte.

In Catalan, the subject pronoun is often left out because the verb ending already tells you who the subject is.

• vull = I want
• The ending itself shows 1st person singular

So Vull fer una sorpresa... is completely natural.
You can say Jo vull... if you want extra emphasis, contrast, or clarity, for example:

• Jo vull fer una sorpresa, no ell.
  I want to do it, not him.

Most of the time, though, just vull is enough.

Yes. Catalan often uses fer in places where English might use a different verb.

• fer una sorpresa = literally to make/do a surprise
• In natural English, we say to surprise someone or to prepare a surprise

But in Catalan, fer una sorpresa is a normal and idiomatic way to say it.

You may also hear:

• preparar una sorpresa = to prepare a surprise

Both are possible, but fer una sorpresa is very common and natural.

That a marks the indirect object.

In this sentence:

• una sorpresa is the thing being made
• a la meva mare tells you for whom or to whom the surprise is directed

So:

• fer una sorpresa a algú = to do/make a surprise for someone

This is very common in Catalan:

• Dono un regal a la meva germana.
• Escric una carta al meu amic.

The a here is not the same as the Spanish-style personal a before direct objects. In this sentence, it is there because la meva mare is functioning as an indirect object.

Catalan normally uses the definite article with possessives:

• la meva mare = my mother
• el meu pare = my father
• la nostra casa = our house

So unlike English, Catalan usually says:

• the my mother literally, although of course that is just normal Catalan structure

This is standard and very important to get used to.

A few family terms can sometimes appear without the article in some contexts, especially in direct address or certain familiar uses, but in a sentence like this, la meva mare is the standard choice.

Because mare is a feminine singular noun.

Catalan possessives agree with the noun they describe:

• meu = masculine singular
• meva = feminine singular
• meus = masculine plural
• meves = feminine plural

So:

• el meu pare
• la meva mare
• els meus amics
• les meves germanes

Since mare is feminine singular, meva is the correct form.

Putting dissabte at the end is a very natural word order in Catalan.

The basic structure is:

• Vull = verb
• fer una sorpresa = what you want to do
• a la meva mare = who it is for
• dissabte = when

Catalan is fairly flexible with word order, but this version sounds natural and straightforward.

You could also move dissabte for emphasis, for example:

• Dissabte vull fer una sorpresa a la meva mare.

That gives a little more prominence to Saturday.

When talking about a specific upcoming day or saying on Saturday, Catalan often just uses the day name without an article:

• dissabte = on Saturday / Saturday

So:

• Vull fer una sorpresa... dissabte.

If you use el dissabte, that often sounds more like on Saturdays or can depend on dialect and context.

Very roughly:

• dissabte = this Saturday / Saturday
• el dissabte = Saturdays, or sometimes a more specific framed use depending on context

For a learner, using dissabte here is the safest and most natural choice.

Yes, absolutely.

That sentence uses a different structure:

• sorprendre algú = to surprise someone directly

So you have two natural options:

• Vull fer una sorpresa a la meva mare dissabte.
• Vull sorprendre la meva mare dissabte.

The first focuses on doing/preparing a surprise.
The second focuses more directly on surprising her.

Both are correct; they just frame the idea a little differently.

Vull is the present tense of voler (to want).

Here is the present tense:

• jo vull
• tu vols
• ell/ella vol
• nosaltres volem
• vosaltres voleu
• ells/elles volen

Even though it is present tense, it often corresponds to English I want to... in exactly the same way:

• Vull fer... = I want to do/make...

After a conjugated verb like vull, Catalan normally uses the infinitive of the next verb.

So:

• vull fer = I want to do/make
• not vull faig

This is the same pattern as in English:

• I want to do
• not I want I do

More examples:

• Vull menjar. = I want to eat.
• Vull anar-hi. = I want to go there.
• Vull veure-la. = I want to see her.

So fer is in the infinitive because it depends on vull.

The order is not completely fixed, but some versions sound more neutral than others.

The original sentence is a natural neutral order:

• Vull fer una sorpresa a la meva mare dissabte.

Other possible orders include:

• Dissabte vull fer una sorpresa a la meva mare.
• A la meva mare, li vull fer una sorpresa dissabte.

These are not identical in tone:

• putting dissabte first emphasizes the time
• putting a la meva mare first can emphasize the person

So yes, Catalan allows movement, but the original is a very good standard version for learners.

Yes, that is also very common.

Catalan often uses a dative clitic like li even when the indirect object is also stated explicitly:

• Li vull fer una sorpresa a la meva mare.

This is called clitic doubling, and it is very natural in spoken Catalan.

So you may see both:

• Vull fer una sorpresa a la meva mare.
• Li vull fer una sorpresa a la meva mare.

The version without li is still understandable and acceptable, especially in simpler learner-style sentences, but with li is often especially natural in real usage.