Au petit-déjeuner, je mange une tartine avec du beurre et de la confiture.

Because au is the mandatory contraction of à + le.

• à + le = au
• petit-déjeuner is masculine: le petit-déjeuner

So au petit-déjeuner literally means at the breakfast meal, but in natural English it is usually translated as at breakfast or for breakfast.

French often uses an article with meals when they are treated as nouns:

• au petit-déjeuner
• au déjeuner
• au dîner

English often drops the article in expressions like for breakfast, at lunch, or at dinner, but French normally keeps it.

As a noun, petit-déjeuner is normally written with a hyphen. It is the standard spelling for breakfast as a meal.

You will usually see:

• le petit-déjeuner = breakfast
• au petit-déjeuner = at breakfast / for breakfast

The phrase Au petit-déjeuner has been moved to the front of the sentence for emphasis or to set the scene.

So the basic sentence is:

• Je mange une tartine avec du beurre et de la confiture.

When the time expression comes first, a comma is often added:

• Au petit-déjeuner, je mange...

It works like English sentences such as In the morning, I drink coffee.

French je mange can mean either one, depending on context.

• I eat = habitual action
• I’m eating = action happening now

In this sentence, because of Au petit-déjeuner, the most natural understanding is often a habitual one: I eat ... for breakfast.

Une tartine is usually a slice of bread with something spread on it, such as butter or jam. It is often not exactly the same as English toast.

A tartine can be:

• plain bread
• toasted bread
• an open-faced slice with toppings

So learners should not assume it always means a fully toasted slice.

Because French is using partitive articles here. They mean some or an unspecified amount of.

• du beurre = some butter
• de la confiture = some jam

This is very common with food and drink when you are talking about an indefinite amount.

If you said le beurre or la confiture, you would usually mean a specific butter or a specific jam already known in the conversation.

The form of the partitive article changes with gender.

• masculine singular noun: du
• feminine singular noun: de la
• before a vowel sound: de l’

So:

• le beurre → du beurre
• la confiture → de la confiture

Normally, no. In French, you usually need an article before nouns in this kind of sentence.

So French prefers:

• avec du beurre et de la confiture

not:

• avec beurre et confiture

English often allows article-less food words more easily than French does.

After ne...pas, partitive articles usually change to de.

For example:

• Je mange du beurre. → Je ne mange pas de beurre.
• Je mange de la confiture. → Je ne mange pas de confiture.

So the negative version would normally be:

• Au petit-déjeuner, je ne mange pas de beurre ni de confiture.

Avec is a very natural way to say that the bread comes with butter and jam.

• une tartine avec du beurre et de la confiture = a slice of bread with butter and jam

Other choices are possible, but they change the feel slightly:

• une tartine de confiture often means a slice of bread spread with jam
• sur emphasizes physical position: du beurre sur la tartine

So avec is simple, neutral, and very natural here.