Demà cal que arribis a temps, perquè l'exposició comença al matí.

Both patterns exist in Catalan, but they mean slightly different things.

• cal + infinitive = a general necessity
  
  • Cal arribar a temps. = It’s necessary to arrive on time.
• cal que + subjunctive = necessity aimed at a specific person or situation
  
  • Cal que arribis a temps. = You need to arrive on time.

In your sentence, the speaker is talking directly to you, so cal que arribis is the natural choice.

Because after cal que, Catalan normally uses the subjunctive.

So:

• cal que arribis
• cal que vinguis
• cal que ho facis

This is very common in Catalan. English does not mark this as strongly, so for an English speaker it can feel unusual. A good rule is:

• cal + infinitive for a general statement
• cal que + subjunctive when a clause with a subject follows

Here, arribis is the present subjunctive form of arribar for tu.

Arribis is the second person singular subjunctive: you arrive / you should arrive.

The present subjunctive of arribar is:

• jo arribi
• tu arribis
• ell/ella/vostè arribi
• nosaltres arribem
• vosaltres arribeu
• ells/elles/vostès arribin

So cal que arribis clearly matches tu.

Cal means something like:

• it is necessary
• one must
• it’s needed

It comes from the verb caldre, but cal is the form you will see most often in everyday Catalan.

Examples:

• Cal estudiar. = You have to study / It’s necessary to study.
• Cal que vinguis. = You need to come.

It is a very useful way to express obligation, often a bit less direct than has de.

Both can mean you have to arrive, but the tone is a little different.

• Has d’arribar a temps = more direct, personal obligation
• Cal que arribis a temps = more impersonal, like it’s necessary that you arrive on time

So:

• has de often sounds like a direct instruction
• cal que sounds a bit more formal or impersonal

In many situations, both are possible.

Catalan often places time expressions at the beginning for emphasis or context, just like English can.

• Demà cal que arribis a temps...
• Cal que arribis a temps demà...

Both are possible, but starting with Demà sets the time immediately: Tomorrow, you need to arrive on time...

This is very natural in Catalan.

A temps means on time or in time.

It is a fixed expression in Catalan:

• arribar a temps = to arrive on time
• ser a temps = to be in time / still have time

You should learn it as a chunk: a temps.

Examples:

• Hem arribat a temps. = We arrived on time.
• Encara som a temps. = We still have time.

Perquè here means because.

The sentence structure is:

• Cal que arribis a temps, perquè l’exposició comença al matí.
• You need to arrive on time, because the exhibition starts in the morning.

A common point for learners is the difference between:

• perquè = because / so that
• per què = why / for what reason

So in this sentence, perquè is one word because it introduces the reason.

L’exposició means the exhibition.

The apostrophe appears because the definite article la is shortened before a vowel:

• la exposició → l’exposició

This is very common in Catalan:

• l’escola
• l’amiga
• l’hotel

So l’ can stand for:

• el before many vowel sounds
• la before many vowel sounds

Here, exposició is feminine, so the full article is la, contracted to l’.

Good question. In Catalan, the present tense is often used for future events when the time is already clear.

So:

• Demà l’exposició comença al matí. means
• Tomorrow the exhibition starts in the morning.

This works because Demà already tells you the action is in the future.

You could also say:

• Demà l’exposició començarà al matí.

That is also correct, but the present tense is very natural for scheduled or planned future events.

Al matí means in the morning.

It is formed from:

• a + el = al

So:

• el matí = the morning
• al matí = in the morning

This is a very common contraction in Catalan:

• a + el = al
• de + el = del

Examples:

• al vespre = in the evening
• al final = in the end / at the end

Sometimes yes, but they are not always identical.

• al matí usually refers to a specific time frame: in the morning
• de matí often has a more habitual or general sense, like in the mornings or during the morning part of the day

In this sentence, comença al matí is the most natural choice because it refers to when the exhibition starts.

A careful approximate pronunciation is:

demà cal que arribis a temps, perquè l’exposició comença al matí

Rough guide:

• Demà ≈ de-MA
• cal que ≈ kal kuh
• arribis ≈ uh-REE-bis
• a temps ≈ uh TEMPS
• perquè ≈ per-KE
• l’exposició ≈ lehks-poh-zee-see-O
• comença ≈ koo-MEN-suh
• al matí ≈ al muh-TEE

A few useful notes:

• the final à / í / ó stress is important
• ll, ny, and open/closed vowels can vary by dialect
• perquè has stress on the last syllable

If you want, I can also break it down in IPA or compare Central Catalan and Valencian pronunciation.

It is neutral and natural, but arribis shows that the speaker is addressing tu, so it is informal singular.

If you wanted a formal version with vostè, you would say:

• Demà cal que arribi a temps, perquè l’exposició comença al matí.

The rest of the sentence stays the same. The only change is the verb form.

Yes, Catalan word order is somewhat flexible.

The original:

• Demà cal que arribis a temps, perquè l’exposició comença al matí.

Other possible versions:

• Cal que arribis a temps demà, perquè l’exposició comença al matí.
• Perquè l’exposició comença al matí, demà cal que arribis a temps.

The original version is probably the most natural in ordinary speech, because it introduces the time first and then gives the instruction and reason clearly.