El dia de l'examen, he d'arribar aviat a l'escola.

It is a time expression. It sets when the action happens: on the day of the exam.

Catalan often puts time phrases at the beginning of a sentence, just like English can do:

• El dia de l'examen, ...
• On the day of the exam, ...

So this first chunk is simply giving the time frame for the rest of the sentence.

Here de means of and links the two nouns:

• el dia = the day
• de l'examen = of the exam

So literally, it is the day of the exam.

This is a very common Catalan pattern:

• el llibre de la Maria = Maria's book / the book of Maria
• la porta de l'escola = the door of the school

Because Catalan articles often drop the vowel and add an apostrophe before a word beginning with a vowel (or silent h).

So:

• el examen becomes l'examen
• la escola becomes l'escola

This is called elision.

Important: l' does not tell you the gender by itself. It can stand for:

• el
  • vowel word
• la
  • vowel word

So in this sentence:

• l'examen is masculine underneath
• l'escola is feminine underneath

Because del is the contraction of de + el, but when the masculine article el comes before a vowel, it changes to l'.

So the steps are:

• de + el examen
• el examen becomes l'examen
• result: de l'examen

So del examen is not the correct form here.

Compare:

• del llibre = of the book
• de l'examen = of the exam

It is the structure haver de + infinitive, which expresses obligation or necessity.

So:

• he = I have
• d' = shortened de
• arribar = to arrive

Together:

• he d'arribar = I have to arrive / I must arrive

This is one of the most important Catalan ways to say have to.

More examples:

• he d'estudiar = I have to study
• hem de marxar = we have to leave
• has de venir = you have to come

No. He here is not the subject pronoun.

It is the first-person singular form of the verb haver in the expression haver de + infinitive.

The subject I is understood from the verb form, so Catalan does not need to say jo unless it wants emphasis.

So:

• he d'arribar = I have to arrive
• jo he d'arribar would only be used for emphasis, and is not the normal neutral version

Catalan often leaves out subject pronouns because the verb already shows the person.

Because de becomes d' before a vowel.

Since arribar begins with a, the vowel in de is dropped:

• de arribar → d'arribar

This is very common in Catalan:

• d'anar
• d'estudiar
• d'obrir

So he d'arribar is just the normal written form.

Because arribar normally uses a before the destination.

So:

• arribar a casa = arrive home
• arribar a l'estació = arrive at the station
• arribar a l'escola = arrive at school

This is similar to English arrive at or arrive to in meaning, although English and Catalan do not always match exactly in preposition use. In Catalan, a is the normal choice here.

Because al is the contraction of a + el, but escola takes the feminine article la, and before a vowel la becomes l'.

So:

• a + la escola
• la escola becomes l'escola
• result: a l'escola

Compare:

• al llibre = to the book
• a l'escola = to the school

Yes. Aviat can mean soon or early, depending on context.

In this sentence, it is understood as early:

• he d'arribar aviat a l'escola = I have to arrive early at school

In other contexts, it can clearly mean soon:

• vindrà aviat = he/she will come soon

So the exact English translation depends on the situation.

Because aviat is an adverb modifying arribar, and this position is very natural in Catalan.

The sentence is structured like this:

• he d'arribar = I have to arrive
• aviat = early
• a l'escola = at/to school

So the sentence flows as:

• I have to arrive early at school

Catalan adverb placement is fairly flexible, but this order sounds normal and clear.

Because the sentence begins with an introductory time phrase, and a comma is often used after that kind of opening element.

So:

• El dia de l'examen, he d'arribar aviat a l'escola.

The comma helps separate the when part from the main statement.

In many cases, the sentence could still be understood without the comma, but using it here is very natural and helps readability.

You cannot tell just from l', because l' can stand for either el or la.

You learn the gender from vocabulary knowledge or from other forms:

• un examen → masculine
• una escola → feminine

Plural forms also help:

• els exàmens = the exams
• les escoles = the schools

So in this sentence:

• examen is masculine
• escola is feminine

In many varieties of Catalan, yes, you may hear haig de. However, he de is the basic standard form to learn for this structure.

So for learners, the safest pattern is:

• he de
• has de
• ha de
• hem de
• heu de
• han de

If you later hear haig de, recognize it as a real variant, but he d'arribar is a perfectly correct standard model.