No parlarem de la festa amb la mare perquè la sorpresa és per al seu aniversari.

Parlarem is the future tense of parlar and means we will talk / we will speak.

• parlem = we talk / we are talking
• parlarem = we will talk

So No parlarem... means We will not talk...

Catalan often uses the future in the same situations where English does, so this is very straightforward here.

Catalan usually makes a sentence negative by putting no directly before the verb:

• Parlarem = we will talk
• No parlarem = we will not talk

This is much simpler than English, because there is no extra do-support like we do not talk.

Because parlar de means to talk about something.

So:

• parlar de una cosa = to talk about something
• parlar de la festa = to talk about the party

This is a very common pattern in Catalan. English uses about, but Catalan uses de.

It can mean either with Mother or to Mother, depending on context, because parlar amb algú can mean both to talk with someone and to talk to someone.

Here, amb la mare means that the mother is the person they are speaking with / speaking to.

So:

• parlar amb algú = to talk with / to someone

In this sentence, No parlarem de la festa amb la mare means they will not discuss the party with her.

Yes. Catalan word order is somewhat flexible.

For example, these are both natural:

• No parlarem de la festa amb la mare.
• No parlarem amb la mare de la festa.

Both mean essentially the same thing: We will not talk to/with Mother about the party.

The version in your sentence is perfectly normal.

Catalan often uses the definite article with family members:

• la mare = Mother / Mom
• el pare = Father / Dad
• la germana = sister
• el germà = brother

This is very natural in Catalan, even where English would often just say Mom or Mother without an article.

So amb la mare is not strange at all; it is standard Catalan.

This is a very common question.

• perquè = because
• per què = why / for what

In your sentence, perquè introduces the reason:

• ...perquè la sorpresa és per al seu aniversari.
• ...because the surprise is for her/his birthday.

So here it has to be perquè as one word.

The accent distinguishes és from es.

• és = is
• es = a reflexive/object pronoun, like himself/herself/itself or part of many verb forms

So in la sorpresa és..., the accent shows that this is the verb ser in the form is.

Per al is the contraction of:

• per a + el = per al

So:

• per al seu aniversari literally means for her/his birthday

This use of per a shows destination, purpose, or intended recipient.

In this sentence, the idea is that the surprise is intended for that occasion: the birthday.

You may hear pel seu aniversari in everyday speech, especially because in many varieties the distinction between per and per a is reduced.

But in careful standard Catalan, per al seu aniversari is very good here because it expresses for in the sense of intended for or for the occasion of the birthday.

So:

• per al seu aniversari = standard and clear
• pel seu aniversari = also heard in real life, depending on dialect and register

Seu means his / her / its / their / your (formal) depending on context.

Catalan possessives agree with the thing possessed, not with the owner’s gender.

Here:

• aniversari is masculine singular
• so the possessive is seu

That does not tell you whether the person whose birthday it is is male or female.

Compare:

• el seu aniversari = his/her birthday
• la seva festa = his/her party

The form changes to match aniversari or festa, not the owner.

Catalan often uses the definite article where English also uses the, and sometimes where English might phrase things a little differently.

Here la sorpresa means the surprise. It refers to a specific surprise already understood in the situation.

So the article is completely natural.

Yes, aniversari is a standard word for birthday in Catalan.

So:

• el seu aniversari = his/her birthday

Depending on region or style, people may express birthday-related ideas in different ways, but aniversari is absolutely normal and correct.

Because Catalan uses articles very regularly.

In this sentence:

• la festa = the party
• la mare = Mother / Mom
• la sorpresa = the surprise
• al seu aniversari = to/for the birthday occasion, with al coming from a + el or here as part of per al

English sometimes drops articles where Catalan keeps them, especially with family members. So this sentence may look article-heavy to an English speaker, but it is very natural Catalan.