Learn Hardcore French: Paul est l'invité principal du festival.

French does not like having two vowel sounds directly next to each other across words.
When le (the) comes before a word that starts with a vowel or silent h, it contracts to l'.

So:

le invité → l'invité (le + invité → l'invité)

This process is called elision.
It happens with le, la, and je, ne, me, te, etc.:

la amie → l'amie
je aime → j'aime

Why do we need the article l' here, when in other sentences like Paul est professeur there is no article?

French treats professions, nationalities, religions differently from most other nouns.

With a profession used in a general sense, you normally omit the article:
- Paul est professeur. (Paul is a teacher.)
- Elle est médecin. (She is a doctor.)
But when you identify someone with a specific role, title, or function in a particular context, you usually use the definite article:
- Paul est le directeur du musée. (Paul is the director of the museum.)
- Paul est l'invité principal du festival. (Paul is the main/lead guest of the festival.)

Here, l'invité principal du festival is a specific, unique role at that festival, so the definite article l' is required.
Saying Paul est invité principal du festival (without the article) sounds wrong to a native speaker.

What exactly does invité principal mean? Is it the same as “special guest” or “guest of honor”?

Literally, invité principal means main guest or principal guest – the most important invited person.

Nuances:

invité principal: neutral, organizational term – the main/lead guest on the program.
invité d'honneur: closer to guest of honor, with a stronger sense of honor/tribute.

Sometimes English special guest overlaps with invité principal, but special guest can be less central than main guest, whereas invité principal suggests the top or leading guest.

Why does the adjective principal come after invité, while in English we say main guest with the adjective before the noun?

In French, most adjectives come after the noun:

une maison rouge (a red house)
un film intéressant (an interesting film)
un invité principal (a main guest)

Some common, short adjectives usually go before the noun (often remembered with the acronym BANGS / BAGS: Beauty, Age, Number, Goodness, Size), like:

un petit livre (a small book)
une belle histoire (a beautiful story)
un vieux film (an old film)

The adjective principal normally goes after the noun:
l'invité principal, le problème principal, la raison principale.

So the word order invité principal is normal French grammar, even though English has main guest.

What does du mean in du festival, and why is it not de le festival?

Du is a contraction of de + le:

de = of, from
le = the (masculine singular)

So:

de + le festival → du festival

This contraction is obligatory in French; you cannot say de le festival.

Compare:

de + le → du
de + les → des
de + la → de la (no change)
de + l' → de l' (no change)

Here du festival means of the festival.

Is there any difference between Paul est l'invité principal du festival and using c'est, like C'est Paul l'invité principal du festival or C'est l'invité principal du festival?

Yes, there are differences in emphasis and structure.

Paul est l'invité principal du festival.
- Neutral, factual statement.
- Subject is Paul.
- You are simply identifying Paul’s role.
C'est Paul l'invité principal du festival.
- Stronger emphasis on Paul (“It is Paul who is the main guest of the festival”).
- Often used to contrast with someone else or to highlight Paul specifically.
C'est l'invité principal du festival.
- Subject is an unspecified it/he/she (often refers to someone just mentioned or pointed at).
- Like saying: He’s the main guest of the festival. / That’s the main guest of the festival.

All are grammatically correct; you choose based on what you want to emphasize.

How do you pronounce the whole sentence?

A careful, standard French pronunciation (IPA) would be:

Paul est l'invité principal du festival.
/ pɔl ɛ l‿ɛ̃.vi.te pʁɛ̃.si.pal dy fɛs.ti.val /

Step by step:

Paul → /pɔl/ (like “pol”)
est → /ɛ/ (like “eh”)
l'invité → /l‿ɛ̃.vi.te/
- in → nasal vowel /ɛ̃/ (air goes through nose; not exactly like English in)
- the liaison between l' and invité makes it sound like one word
principal → /pʁɛ̃.si.pal/
du → /dy/ (like “dew” with rounded lips)
festival → /fɛs.ti.val/

There is a liaison in l'invité, but no required liaison between principal and du in normal speech.

What is the gender of invité and festival, and does it affect the sentence?

invité here is masculine (the invited person is male: Paul).
- masculine singular: un invité, l'invité
- feminine singular: une invitée, l'invitée (spelled with e at the end, but pronounced the same)
festival is also masculine:
- un festival, le festival, du festival

The gender affects:

the choice of article: l'invité, du festival (de + le → du)
the adjective agreement:
- masculine: invité principal
- feminine: invitée principale

So with a woman, you would write:
Marie est l'invitée principale du festival.

Could you say Paul est un invité principal du festival? What would be the difference?

You can say Paul est un invité principal du festival, but the meaning changes:

Paul est l'invité principal du festival.
- There is one main guest, and it is Paul. (unique role)
Paul est un invité principal du festival.
- Paul is one of several main/key guests.
- Suggests that the festival has multiple invités principaux (for example, several big names at the same level).

So le / l' identifies a single, unique main guest; un makes Paul one among others.

Why is festival not capitalized, and is that normal in French?

In French, common nouns are generally not capitalized, even when they refer to important events or institutions:

le festival de Cannes (Cannes Film Festival)
l'université de Paris (University of Paris)
le musée du Louvre

You capitalize:

proper names: Paul, Cannes, Louvre
the first word of a sentence

So in Paul est l'invité principal du festival., only Paul (proper name) and the first word of the sentence are capitalized; festival stays in lowercase.