Meine Nichte mag Müsli nicht, aber Quark mit Honig isst sie gern.

Because Nichte is a feminine noun in German.

• die Nichte = the niece
• mein changes its ending to match the noun
• for a feminine singular noun in the nominative, you use meine

So:

• meine Nichte = my niece

Here, meine Nichte is also the subject of the sentence.

The verb is mögen = to like.

It is conjugated to match the subject meine Nichte, which is third-person singular (she):

• ich mag
• du magst
• er/sie/es mag

So:

• Meine Nichte mag ... = My niece likes ...

They are both in the accusative, because they are the direct objects of the verbs:

• mag Müsli
• isst Quark mit Honig

In this sentence, you do not see a special accusative ending, because:

• Müsli and Quark are used without articles
• many German nouns do not visibly change in the accusative by themselves

So the case is determined by the sentence structure, not by a visible noun ending here.

German often leaves out the article with foods when speaking in general.

So:

• Müsli = muesli, as a general food
• Quark mit Honig = quark with honey, as a general food combination

This is similar to English when we say:

• I like coffee
• She eats yogurt

You could use an article if you mean something specific:

• das Müsli auf dem Tisch = the muesli on the table
• den Quark aus dem Kühlschrank = the quark from the fridge

German has more than one natural way to express liking.

1. etwas mögen = to like something

   • Sie mag Müsli nicht. = She doesn’t like muesli.

2. etwas gern essen = to like eating something / to enjoy eating something

   • Quark mit Honig isst sie gern. = She likes eating quark with honey.

So both are natural, but they are built differently:

• mögen is a main verb meaning to like
• gern is an adverb meaning gladly / with pleasure, and with a verb it often corresponds to English like to

You could also say:

• Meine Nichte mag Quark mit Honig.

That would also be correct.

In Meine Nichte mag Müsli nicht, the nicht negates the idea of liking Müsli.

German often places nicht fairly late in the clause, especially when it negates the verb phrase or the object idea as a whole.

So:

• Sie mag Müsli nicht. = She doesn’t like muesli.

If you moved nicht, the emphasis could change and it might sound contrastive or unnatural in this context.

This sentence uses the normal, natural order.

Because German allows you to put different elements in the first position of a main clause for emphasis or contrast.

Here, Quark mit Honig is moved to the front to contrast it with Müsli:

• Meine Nichte mag Müsli nicht, aber Quark mit Honig isst sie gern.

This highlights the contrast:

• not Müsli
• but Quark mit Honig

A more neutral order would be:

• ..., aber sie isst Quark mit Honig gern.

Both are correct, but the original version emphasizes the food.

Because German main clauses follow the verb-second rule.

That means the finite verb must be in the second position of the clause.

In the second clause, the first position is already taken by Quark mit Honig, so the verb must come next:

• Quark mit Honig | isst | sie | gern

So once the object is moved to the front, the subject sie comes after the verb.

This is called inversion in many learner explanations.

gern usually comes later in the clause, often after the object in a simple sentence.

So:

• Quark mit Honig isst sie gern.

This is a very natural placement.

In a more neutral sentence, you will also often see:

• Sie isst gern Quark mit Honig.

Both are correct. The difference is mainly about focus and word order, not basic meaning.

The preposition mit always takes the dative case.

So:

• mit Honig = with honey

Here, you do not see a special dative ending because Honig has no article in this phrase.

If there were an article, you would see the dative more clearly:

• mit dem Honig

So mit Honig is a prepositional phrase in the dative, and it describes the Quark.

Because aber joins two main clauses here:

• Meine Nichte mag Müsli nicht
• aber Quark mit Honig isst sie gern

In German, a comma is normally used before aber in this kind of sentence.

So the comma helps separate the two full clauses and makes the contrast clear.

Because aber expresses a general contrast, while sondern is used to correct a previous negative statement.

Here, the sentence means:

• she does not like one thing
• but she does like another thing

That is a contrast, so aber fits well.

sondern would be more appropriate in a sentence like:

• Sie isst nicht Müsli, sondern Quark mit Honig.
• She doesn’t eat muesli, but rather quark with honey.

That version directly replaces one option with the other. The original sentence is a bit broader: it talks about not liking one food and liking to eat another.