Je dois t’avouer quelque chose : je préfère la brioche au croissant.

Because devoir works directly with an infinitive: devoir + infinitive.

So you say:

• je dois partir = I must leave
• tu dois travailler = you must work
• je dois t’avouer = I must confess to you

There is no de after devoir in this structure.

T’ is the shortened form of te, meaning to you here.

The full version would be je dois te avouer, but French avoids te before a vowel sound, so it becomes t’:

• te + avouer → t’avouer

It comes before the infinitive because French object pronouns usually go before the verb they belong to:

• je vais te parler
• je peux te dire
• je dois t’avouer

So t’avouer means to confess to you.

Quelque chose means something.

It is a fixed expression:

• quelque chose = something
• quelqu’un = someone

If you said une chose, that would mean a thing, which sounds much more literal and less natural in this sentence.
So je dois t’avouer quelque chose is the normal way to say I have to confess something to you.

The colon introduces what that something is.

So the sentence works like this:

• Je dois t’avouer quelque chose = I have to confess something to you
• : je préfère la brioche au croissant = namely, I prefer brioche to croissants

It is similar to English using a colon before the explanation or revelation.

Also, in standard French typography, a space is usually placed before a colon: quelque chose :

Because préfère is the conjugated form, while préférer is the infinitive.

• préférer = to prefer
• je préfère = I prefer

This verb is a stem-changing verb. In the singular forms and the ils/elles form, the second é often changes to è:

• je préfère
• tu préfères
• il/elle préfère
• nous préférons
• vous préférez
• ils/elles préfèrent

So je préfère is the correct present-tense form.

French usually uses the definite article when talking about things in general, including food.

So:

• j’aime le café = I like coffee
• elle adore le chocolat = she loves chocolate
• je préfère la brioche au croissant = I prefer brioche to croissants / to the croissant

In English, we often drop the article in general statements, but French usually keeps it.

Here la brioche and le croissant refer to those foods as categories, not one specific brioche and one specific croissant.

Because à + le contracts to au.

So:

• à + le = au
• à + les = aux

That means:

• je préfère la brioche à le croissant → je préfère la brioche au croissant

This à is the one used with préférer in the pattern préférer A à B = to prefer A to B.

In French, one common pattern is:

préférer A à B = to prefer A to B

So:

• Je préfère le thé au café
• Elle préfère Paris à Londres
• Je préfère la brioche au croissant

In English, we often simply say I prefer brioche over croissants or just I prefer brioche, but French often marks both elements clearly with à when comparing two things.

Yes, you could. That is also grammatical.

But there is a small difference in feel:

• préférer A à B is the classic, standard comparison structure
• préférer A plutôt que B is also common and can sound a little more explicit or emphatic

So both are possible, but je préfère la brioche au croissant is very natural and elegant.

Not necessarily. Avouer literally means to confess or to admit, so it can sound more dramatic than dire.

That is exactly why it is funny or expressive here: the speaker treats a pastry preference like a big secret.

So:

• je dois te dire quelque chose = I need to tell you something
• je dois t’avouer quelque chose = I have to confess something to you

The second one adds a playful or dramatic tone in this sentence.

Yes. It is a very natural way to introduce a confession, revelation, or even a joke.

French speakers often use structures like:

• je dois t’avouer quelque chose
• il faut que je t’avoue quelque chose
• je vais te dire un secret

In your sentence, that suspense makes the ending amusing, because the big confession is simply a preference for brioche over croissant.