Zum Frühstück esse ich Müsli mit Quark, Trauben und Kirschen.

Zum is a contraction of zu dem.

• zu = a preposition
• dem = the dative form of das
• das Frühstück is neuter, so after zu it becomes dem Frühstück

So:

• zu dem Frühstück → zum Frühstück

In this sentence, zum Frühstück is the normal German way to say for breakfast or at breakfast.

Because Frühstück is a noun, and all German nouns are capitalized.

That is true even when the word would not be capitalized in English.

Compare:

• das Frühstück = the breakfast → capitalized
• frühstücken = to eat breakfast → not capitalized

In a German main clause, the finite verb normally has to be in second position.

Here, Zum Frühstück has been placed first. That means the verb esse must come next, and the subject ich comes after it:

• Zum Frühstück | esse | ich ...

This is completely normal German word order.

Yes, absolutely.

Both are correct:

• Zum Frühstück esse ich ...
• Ich esse zum Frühstück ...

The difference is mainly emphasis:

• Zum Frühstück esse ich ... puts the breakfast time first
• Ich esse zum Frühstück ... starts more neutrally with I

Because esse is the 1st person singular form of the verb essen.

The present tense forms are:

• ich esse
• du isst
• er / sie / es isst
• wir essen
• ihr esst
• sie / Sie essen

So ich esse means I eat.

German often omits articles with food when speaking in a general way.

So Ich esse Müsli sounds natural, just like English often says I eat cereal rather than I eat a cereal.

The same idea applies here:

• Müsli
• Quark
• Trauben
• Kirschen

If you wanted to be more specific, you could use an article or a quantity word, for example:

• ein Müsli
• den Quark
• ein paar Trauben
• die Kirschen

But in this sentence, no article is perfectly normal.

Mit always takes the dative case.

So grammatically, these are in the dative:

• mit Quark
• mit Trauben
• mit Kirschen

Why do they look unchanged?

• Quark has no visible article here, so you do not see the dative clearly
• Trauben and Kirschen are plural nouns that already end in -n, so the dative plural looks the same

If you included articles, the dative would be more obvious:

• mit dem Quark
• mit den Trauben
• mit den Kirschen

Because one preposition can introduce a whole list.

So:

• mit Quark, Trauben und Kirschen

means:

• with quark, grapes, and cherries

You do not need to repeat mit before each item.

You could repeat it for style or emphasis, but normally German uses just one:

• mit Quark, mit Trauben und mit Kirschen → possible, but less natural here

Quark is a very common dairy product in German-speaking countries.

For an English speaker, it is best thought of as something like:

• a fresh soft cheese
• somewhere between yogurt and cream cheese, depending on the type

It is not the same as the English science word quark.

Because the sentence is talking about grapes and cherries in general, not just one grape or one cherry.

The singular forms are:

• die Traube = grape
• die Kirsche = cherry

The plural forms are:

• die Trauben = grapes
• die Kirschen = cherries

That matches the idea of adding fruit to muesli: you would normally use more than one.

The hardest part for many English speakers is ü.

A helpful way to approach ü is:

1. Make an ee sound
2. Keep your tongue there
3. Round your lips

Approximate pronunciations:

• Müsli ≈ MÜS-lee
• Frühstück ≈ FRÜÜ-shtük

A couple of extra notes:

• The st in Stück is pronounced sht
• The ü sound has no exact English equivalent

So the main thing to practice is the vowel sound in Mü- and Früh-.