Ich mag knusprige Brötchen, aber meine Freundin isst lieber Melone zum Frühstück.

Because Brötchen is plural here, and there is no article before it.

In German, adjective endings depend on:

• gender
• case
• number
• whether there is a determiner like der/ein/mein

Here:

• Brötchen is the direct object of mag, so it is in the accusative
• it is plural
• there is no article

With an adjective before a noun in the accusative plural with no article, the adjective takes -e:

• ich mag knusprige Brötchen

So knusprige is correct.

Yes. -chen is a diminutive ending, meaning something like little or small.

• das Brot = bread
• das Brötchen = little bread roll / bread roll

A very important point: nouns ending in -chen are always neuter and often have the same form in singular and plural.

So:

• das Brötchen = the bread roll
• die Brötchen = the bread rolls

In your sentence, the meaning is plural because the speaker likes bread rolls in general, not just one.

Because aber is a coordinating conjunction. It does not push the verb to the end.

German main clauses usually follow the verb-second pattern:

• Meine Freundin isst lieber Melone.

When you add aber, the next clause is still a normal main clause:

• ..., aber meine Freundin isst lieber Melone ...

Compare that with a subordinating conjunction like weil, which sends the verb to the end:

• ..., weil meine Freundin lieber Melone isst.

So after aber, normal word order stays.

Here lieber means preferably or rather.

It comes from gern:

• gern = gladly / like doing
• lieber = prefer / rather
• am liebsten = most of all

So:

• Sie isst gern Melone. = She likes eating melon.
• Sie isst lieber Melone. = She prefers eating melon.

In this sentence, lieber shows a preference:

• the speaker likes crispy rolls,
• but the girlfriend prefers melon for breakfast.

German often leaves out the article when talking about food in a general sense, especially after verbs like essen, trinken, kaufen, etc.

So:

• Sie isst Melone. = She eats melon.
• Sie trinkt Kaffee. = She drinks coffee.

This sounds natural when you mean the food as a type of thing, not one specific item.

If you add an article, the meaning becomes more specific:

• Sie isst eine Melone. = She is eating a whole melon / one melon.
• Sie isst die Melone. = She is eating the specific melon.

So in your sentence, Melone is used in a general, natural food-related way.

Zum is a contraction of zu dem:

• zu dem Frühstück → zum Frühstück

Literally, zum Frühstück means something like to breakfast, but in natural English it means:

• for breakfast
• at breakfast

So:

• Melone zum Frühstück = melon for breakfast

This contraction is very common in German:

• zum Bahnhof = to the train station
• zum Arzt = to the doctor
• zum Frühstück = for breakfast

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

In this sentence, the capitalized nouns are:

• Brötchen
• Freundin
• Melone
• Frühstück

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

Because Freundin is a feminine noun.

The basic nominative forms of mein- are:

• mein for masculine/neuter singular
• meine for feminine singular
• meine for plural

Since Freundin is feminine and is the subject of the second clause, you need:

• meine Freundin

Examples:

• mein Freund = my boyfriend / male friend
• meine Freundin = my girlfriend / female friend

They express two slightly different ideas:

• ich mag knusprige Brötchen = I like crispy bread rolls
• meine Freundin isst lieber Melone = my girlfriend prefers eating melon

So:

• mögen is used for liking a noun or thing
• lieber with a verb shows preference

You could also say:

• Ich esse gern knusprige Brötchen. = I like eating crispy bread rolls.

That would be closer in structure to the second clause. But ich mag knusprige Brötchen is perfectly natural.

It can mean girlfriend or female friend, depending on context.

• meine Freundin often means my girlfriend
• but it can also mean my female friend

In real life, context usually makes it clear. Since your English meaning has already been shown to the learner, the intended meaning is already known there.

A related pair:

• mein Freund = my boyfriend / my male friend
• meine Freundin = my girlfriend / my female friend

Yes. Aber means but, and it introduces a contrast.

Here the contrast is:

• the speaker likes crispy bread rolls
• the girlfriend prefers melon for breakfast

So aber connects two different preferences.

It is a very common word and does not usually sound dramatic by itself. It just marks a simple contrast:

• Ich trinke Kaffee, aber sie trinkt Tee.
• I drink coffee, but she drinks tea.

A few helpful pronunciation notes:

• ich: the ch is the soft German ich-Laut, not like English k or ch in chair
• knusprige: the kn is pronounced with both sounds; the g at the end of -ig is often pronounced somewhat like -ich in many accents
• Brötchen:
  • ö is like a rounded vowel not found naturally in standard English
  • ch again is the soft sound
  • -chen is pronounced roughly like hyen or khen only approximately, but not exactly like any normal English spelling

A rough English-friendly approximation might be:

• ich ≈ ikh/ish but softer
• Brötchen ≈ BRURT-hyen very approximately

The exact sounds are best learned by listening to native audio.

Yes, that is possible, but it sounds a bit less natural than isst lieber in this context.

• mag lieber Melone = likes melon better / prefers melon
• isst lieber Melone = prefers eating melon

Because the sentence is about breakfast, using isst makes the action clearer and more idiomatic.

So both are possible, but:

• meine Freundin isst lieber Melone zum Frühstück sounds especially natural.