Meine Schwester isst Quark mit Himbeeren und Blaubeeren.

Because meine Schwester is the subject of the sentence, so it is in the nominative case.

• Schwester is a feminine noun.
• meine is the form of mein used for feminine singular nominative.

So:

• meine Schwester = my sister as the subject
• meiner Schwester would usually be used in dative or genitive contexts, not here

Because the subject is Meine Schwester, which is third person singular: she.

The verb essen changes like this in the present tense:

• ich esse = I eat
• du isst = you eat
• er/sie/es isst = he/she/it eats
• wir essen = we eat
• ihr esst = you all eat
• sie/Sie essen = they/you eat

So with meine Schwester (she), the correct form is isst.

Also note that essen is one of those verbs that changes its stem vowel in du and er/sie/es forms:

• e → i

In this sentence, Quark means a soft fresh dairy product, somewhat similar to:

• strained yogurt
• fromage blanc
• a very mild fresh cheese

It does not mean the English word quark from physics.

In German-speaking countries, Quark is a very common food, often eaten:

• plain
• with fruit
• with sugar or honey
• in desserts
• on bread

So in this sentence, it is the food item.

Because Quark is being used as a mass noun, like bread, rice, or cheese in English.

German often leaves out the article with food when talking about it in a general, non-specific way.

So:

• Meine Schwester isst Quark. = My sister is eating quark.

You could also say:

• Meine Schwester isst den Quark.

But that would usually mean a specific quark, for example one already mentioned or one sitting on the table.

Without the article, it sounds more general and natural here.

Because mit is the preposition meaning with, and in German prepositions normally come before the noun phrase they belong to.

So:

• mit Himbeeren und Blaubeeren = with raspberries and blueberries

This works very much like English in this case.

Yes. Mit always takes the dative case.

That means the nouns after mit are in the dative.

Here, though, you do not see a visible change, because:

• Himbeeren and Blaubeeren are plural
• there is no article
• many plural noun forms look the same in nominative and dative

If you added an article, the dative would be clearer:

• mit den Himbeeren und den Blaubeeren

So the rule is still there: mit + dative.

Because German often uses bare plurals when speaking generally about plural things, especially ingredients or foods.

So:

• mit Himbeeren und Blaubeeren = with raspberries and blueberries

This sounds natural when you mean berries in a general sense.

If you meant specific berries, you could use articles:

• mit den Himbeeren und den Blaubeeren = with the raspberries and the blueberries

But in a simple food sentence like this, leaving out the articles is very normal.

This is a very standard German main-clause word order pattern:

• Meine Schwester = subject
• isst = verb
• Quark = object
• mit Himbeeren und Blaubeeren = prepositional phrase

German main clauses usually put the finite verb in the second position.

So the structure is basically:

Subject + verb + rest

That is why isst comes right after Meine Schwester.

Because in German, all nouns are capitalized, not just proper names.

So in this sentence:

• Schwester
• Quark
• Himbeeren
• Blaubeeren

are all capitalized because they are nouns.

This is one of the most noticeable spelling differences between German and English.

isst is pronounced roughly like ist, because the double s here just represents the normal s sound after a short vowel.

A rough guide:

• isst ≈ ist in pronunciation
• it sounds something like ist with a short i

Also, learners often confuse:

• ist = is
• isst = eats

They sound the same in standard pronunciation, so you tell them apart from context.

Both words can mean blueberries.

• Blaubeeren is very common and easy to understand everywhere.
• Heidelbeeren is also common and may sound a bit more formal or regional depending on context.

So in this sentence, Blaubeeren is completely normal.

Yes. Mit is fairly broad.

In this sentence, it most naturally means that the quark is eaten with raspberries and blueberries, which could imply:

• served alongside them
• topped with them
• mixed with them

The exact image depends on context. German does not force a more specific interpretation here.

Yes, absolutely. That is a normal way to list more fruits.

German uses und before the last item in a list, just like English:

• mit Himbeeren, Blaubeeren und Erdbeeren

If you want, you can keep adding ingredients in the same pattern.