Am Morgen isst meine Mutter Müsli mit einer Banane, aber ich trinke nur Kaffee.

Am Morgen means in the morning.

Here, am is a contraction of an dem. In German, certain time expressions are just idiomatic this way:

• am Morgen = in the morning
• am Abend = in the evening
• am Montag = on Monday

So this is simply the normal German expression for that time of day.

German often puts a time expression first to set the scene. That is very natural.

The important grammar point is this: in a main clause, the finite verb must be in second position.

So:

• Am Morgen = position 1
• isst = position 2
• meine Mutter = comes after the verb

That is why it is:

• Am Morgen isst meine Mutter ...

and not:

• Am Morgen meine Mutter isst ...

Because of the German verb-second rule.

If the sentence started with the subject, you would get:

• Meine Mutter isst am Morgen Müsli ...

But once Am Morgen is moved to the front, the verb must still stay in second position:

• Am Morgen isst meine Mutter ...

So the subject moves after the verb.

Because meine Mutter is the subject of the clause, so it is in the nominative case.

For a feminine noun like Mutter, the possessive mein- becomes:

• meine Mutter = nominative / accusative
• meiner Mutter = usually dative or genitive, depending on context

Here, she is the one doing the action of eating, so nominative is needed:

• meine Mutter isst

Those are different verb forms because they match different subjects.

• meine Mutter isst → sie eats → 3rd person singular
• ich trinke → I drink → 1st person singular

Also, essen is a strong/irregular verb:

• ich esse
• du isst
• er/sie/es isst

Notice the vowel change from e to i in isst.

But trinken is regular:

• ich trinke
• du trinkst
• er/sie/es trinkt

In German, as in English, food and drink often appear without an article when you mean them in a general, indefinite way.

So:

• Müsli = muesli/cereal
• Kaffee = coffee

This is similar to English:

• She eats cereal.
• I drink coffee.

If you wanted to refer to a specific one, an article could appear:

• das Müsli = the muesli
• einen Kaffee = a coffee / one coffee

But in this sentence, the general non-specific form is natural.

Because the preposition mit always takes the dative case.

The noun Banane is feminine, and the feminine indefinite article in the dative is einer:

• nominative: eine Banane
• accusative: eine Banane
• dative: einer Banane

So:

• mit einer Banane = with a banana

Because the sentence says she eats muesli with one banana.

If it were plural, it would be:

• mit Bananen = with bananas

So the singular tells you the speaker has one banana in mind, or is describing a typical serving that includes a banana.

Nur means only or just.

In this sentence, it limits Kaffee, so:

• ich trinke nur Kaffee = I drink only coffee

Placing nur before the word or phrase it focuses on is very common in German.

Compare:

• Ich trinke nur Kaffee. = coffee and nothing else
• Ich trinke Kaffee nur am Morgen. = I drink coffee only in the morning

So word placement changes what nur emphasizes.

Because aber is a coordinating conjunction. It connects two main clauses, but it does not force the verb to the end, and it does not count as position 1 in the clause.

So after aber, normal main-clause word order continues:

• aber ich trinke nur Kaffee

If you wanted trinke before ich, something else would need to be in first position, for example:

• ... aber am Morgen trinke ich nur Kaffee.

So aber does not itself trigger inversion.

Because aber connects two full main clauses:

• Am Morgen isst meine Mutter Müsli mit einer Banane
• ich trinke nur Kaffee

In German, a comma is normally used before coordinating conjunctions like aber when they join two independent clauses.

Yes. Both can work, but they are used slightly differently.

• Am Morgen = in the morning, referring to the morning as a time period
• morgens = in the mornings / every morning, often more habitual

So:

• Am Morgen isst meine Mutter Müsli ... sounds like in the morning
• Morgens isst meine Mutter Müsli ... can sound more like a routine: in the mornings

In many contexts, both are natural.

Because in German, all nouns are capitalized.

That is a basic spelling rule, so you will see:

• der Morgen
• die Mutter
• das Müsli
• die Banane
• der Kaffee

This is different from English, where common nouns are normally lowercase.