Domenica mattina la brioche è più buona se la mangio ancora calda.

Italian often uses time expressions without a preposition where English needs one.

So:

• domenica mattina = on Sunday morning
• lunedì sera = on Monday evening
• martedì pomeriggio = on Tuesday afternoon

A useful contrast is:

• Domenica mattina = on Sunday morning / this Sunday morning / Sunday morning depending on context
• La domenica mattina = on Sunday mornings / every Sunday morning

Also, days of the week are normally lowercase in Italian. It is capitalized here only because it begins the sentence.

They are doing two different jobs:

• la brioche: here la is the definite article, meaning the
• la mangio: here la is a direct object pronoun, meaning it

So:

• la brioche = the brioche
• la mangio = I eat it

Italian often avoids repeating the noun if it has already been mentioned, so instead of repeating la brioche, it uses la.

Yes. In Italian, brioche is treated as a feminine singular noun, so it takes feminine forms:

• la brioche
• più buona
• calda

This is something you mostly have to learn with the noun, because borrowed words do not always have endings that clearly show gender.

Because Italian usually leaves out subject pronouns when the verb ending already makes the subject clear.

• mangio already means I eat
• mangi means you eat
• mangia means he/she eats

So se la mangio naturally means if I eat it.

You can add io, but usually only for emphasis or contrast:

• se io la mangio = if I eat it and not someone else

Because buona is an adjective, while bene is an adverb.

After essere, Italian normally uses an adjective:

• è buona = it is good / tasty
• è calda = it is warm

Bene is used with verbs, not to describe a noun after essere:

• cucina bene = he/she cooks well

So with food, you say:

• La brioche è buona not
• La brioche è bene

Both can be possible in some situations, but più buono/a is especially common when talking about taste.

With food:

• più buona often means tastier
• migliore can sound more general, like better overall or of better quality

So la brioche è più buona sounds very natural if you are talking about how good it tastes.

Calda refers to la brioche, not to the speaker.

So the meaning is:

• I eat it while it is still warm

Because brioche is feminine singular, the adjective must agree with it:

• la brioche → calda
• il panino → caldo

And ancora here means still, so:

• ancora calda = still warm

Yes, that is grammatical.

It would mean essentially the same thing:

• se la mangio ancora calda
• se mangio la brioche ancora calda

The version with la is usually more natural here because the noun la brioche was already mentioned. Italian often uses a pronoun instead of repeating the noun again right away.

Because unstressed direct object pronouns normally come before a conjugated verb in Italian.

So:

• la mangio = I eat it
• lo vedo = I see him/it
• ti chiamo = I call you

This is one of the big differences from English, where the object usually comes after the verb.

Italian uses the present tense very often for general truths, habits, and things that are generally true.

So this sentence is not necessarily about one single moment only. It can mean something like:

• Brioche tastes better if I eat it while it is still warm

That is a general statement, so the present tense fits perfectly.

Because se introduces a condition:

• è più buona se la mangio ancora calda = it is better if I eat it still warm

Quando means when, and it sounds more factual or habitual:

• Quando la mangio ancora calda, è più buona = When I eat it still warm, it is better

Both can make sense, but se is the natural choice for a basic if condition.

Not always.

In Italian, brioche often refers to a sweet breakfast pastry, and in many contexts it can be closer to what an English speaker might think of as a croissant or a filled breakfast pastry, depending on the region.

So the word is familiar, but its everyday use in Italian may be a bit broader or different from English.