Dilluns vaig al teatre amb la meva mare perquè hi fan una funció nova.

Both are possible, but they do not always feel exactly the same.

• Dilluns = Monday / on Monday
• el dilluns can mean on Monday, but it is also often used for Mondays in general

In this sentence, Dilluns most naturally means on Monday (a specific upcoming Monday).

Examples:

• Dilluns vaig al teatre. = On Monday I’m going to the theatre.
• El dilluns vaig al teatre. = can also mean On Monday I go/am going to the theatre
• Els dilluns vaig al teatre. = On Mondays I go to the theatre

Also, weekdays are normally lowercase in Catalan, so dilluns is only capitalized here because it is at the beginning of the sentence.

This is a very common point of confusion.

By itself, vaig is the present tense of anar (to go):

• jo vaig = I go

But Catalan also has a very common past tense called the periphrastic past, which uses:

• vaig + infinitive = I did / I went / I ...-ed

So:

• vaig al teatre = I go / I’m going to the theatre
• vaig anar al teatre = I went to the theatre

In your sentence, there is no infinitive after vaig, so it is simply the present tense: I go / I’m going.

Catalan, like English, often uses the present tense for a planned or scheduled future event.

So:

• Dilluns vaig al teatre literally looks like On Monday I go to the theatre
• but in natural English it is usually On Monday I’m going to the theatre

This use of the present makes the plan sound definite or arranged.

Other examples:

• Demà treballo. = Tomorrow I’m working.
• Aquest vespre sopem fora. = This evening we’re eating out.

So this is completely normal Catalan.

Catalan often drops subject pronouns when they are not needed, because the verb ending already shows who the subject is.

• (Jo) vaig = I go
• (Tu) vas = you go
• (Ell/ella) va = he/she goes

So jo is optional here. You would include it mainly for emphasis, contrast, or clarity.

For example:

• Vaig al teatre. = I’m going to the theatre.
• Jo vaig al teatre, però ella es queda a casa. = I’m going to the theatre, but she’s staying home.

Because a + el contracts to al in Catalan.

So:

• a + el = al
• de + el = del

Examples:

• Vaig al teatre. = I’m going to the theatre.
• Vinc del teatre. = I’m coming from the theatre.

This is very similar to other contractions in Romance languages.

Note that this only happens with the masculine singular article el:

• a la platja = no contraction
• a l’escola = before a vowel
• als teatres = a + els

Catalan usually uses the definite article with possessives:

• el meu pare = my father
• la meva mare = my mother
• els meus amics = my friends

So la meva mare is the normal way to say my mother.

This is especially common with family members and many everyday nouns.

A few kinship terms can sometimes appear without the article in certain contexts, especially in older, literary, or more fixed usage, but for learners, the safest and most natural pattern is:

• el meu... / la meva... / els meus... / les meves...

Here perquè means because.

Catalan distinguishes between forms like these:

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

So in your sentence:

• ...perquè hi fan una funció nova. = ...because they’re putting on a new show there.

The accent helps distinguish it from other similar-looking forms.

Examples:

• No vinc perquè estic cansat. = I’m not coming because I’m tired.
• Per què no véns? = Why aren’t you coming?

Here hi means there, and it refers back to the theatre.

So:

• hi fan una funció nova = they’re putting on a new performance there

Catalan often uses hi as a locative pronoun to avoid repeating a place.

Compare:

• Vaig al teatre perquè al teatre fan una funció nova.
• More natural: Vaig al teatre perquè hi fan una funció nova.

So hi replaces al teatre / allí in this context.

You will see hi a lot in Catalan. It can stand for:

• there
• in it / at it / to it depending on context.

In this sentence, fan is an impersonal/general plural. It does not necessarily refer to a specific named group.

• fan literally = they do / they make
• but in this context it means something like they are putting on, they are showing, or they have on

So:

• hi fan una funció nova = they’re putting on a new performance there

The implied subject could be:

• the theatre
• the people who run it
• the company performing there
• the organizers

Catalan often uses the third person plural this way, just like English sometimes says they without naming exactly who they are.

Funció can mean performance, show, or staging, depending on context.

In a theatre sentence, una funció nova most naturally means:

• a new performance
• a new show
• possibly a new production

It does not usually mean function in the mathematical or technical English sense here, even though the words look related.

So:

• fan una funció nova = they’re putting on a new show / performance

In Catalan, adjectives often come after the noun:

• una funció nova = a new performance
• una casa gran = a big house
• un llibre interessant = an interesting book

This is the most neutral and common order.

Sometimes an adjective can go before the noun, but that may change the tone, emphasis, or meaning. For learners, the basic rule is:

• noun + adjective is very common and usually the safest choice

So una funció nova is the normal, natural wording.

Not naturally.

A more natural English translation would be:

• On Monday I’m going to the theatre with my mother because they’re putting on a new show there.
• or ...because there’s a new performance there.

The literal word-by-word route can be misleading:

• vaig here is present, but often translated as I’m going
• fan is not usually do in natural English here
• funció is not usually function in this context
• hi becomes there

So it is better to translate for meaning, not word for word.

This particular sentence most naturally suggests a specific Monday, especially because it begins with Dilluns and continues with a reason for that plan.

However, without more context, the present tense can sometimes describe routine actions too.

To make the habitual meaning clearer, Catalan would more often say:

• Els dilluns vaig al teatre amb la meva mare... = On Mondays I go to the theatre with my mother...

So:

• Dilluns... = usually on Monday (one occasion)
• Els dilluns... = on Mondays (habitually)

That distinction is very useful to remember.