Το αφρόγαλα στον καπουτσίνο είναι ωραίο, αλλά εγώ προτιμώ τον καφέ πιο απλό.

It is a compound word:

• αφρός = foam
• γάλα = milk

So αφρόγαλα literally means foam-milk, i.e. milk foam / frothed milk. In coffee contexts, it is the foamy milk on top of a cappuccino.

Because some borrowed drink names in Greek are treated as masculine, even if their ending might look neuter to a learner.

Here the sentence uses:

• ο καπουτσίνο = the cappuccino
• τον καπουτσίνο = the cappuccino, as object or after a preposition

This is common with coffee names in everyday Greek. The noun itself often does not change much, so the article shows the gender and case.

Στον is the contracted form of σε τον.

So:

• σε = in, at, on, to
• τον = the, masculine accusative singular
• στον = in the / to the / at the, depending on context

Here στον καπουτσίνο means in the cappuccino.

Greek uses the definite article much more often than English.

So Greek may say:

• Το αφρόγαλα where English would just say milk foam or the milk foam
• τον καφέ where English might just say coffee

In this sentence, the articles sound natural in Greek even though English would often leave them out.

Because ωραίο agrees with το αφρόγαλα.

• αφρόγαλα is neuter singular
• so the adjective must also be neuter singular

That gives:

• ωραίος = masculine
• ωραία = feminine
• ωραίο = neuter

So Το αφρόγαλα ... είναι ωραίο means The milk foam ... is nice.

Yes. The ending of προτιμώ already tells you the subject is I.

So εγώ is not required for basic grammar. It is added for emphasis or contrast.

Here, after αλλά, it means something like:

• but I prefer...
• but as for me, I prefer...

So εγώ adds a contrast with the first part of the sentence.

Because καφές is the subject form (nominative), while καφέ here is the object form (accusative).

After προτιμώ, the thing preferred is the direct object, so Greek uses the accusative:

• ο καφές = the coffee
• τον καφέ = the coffee, as object

This is a very common pattern with masculine nouns in -ης:

• ο καφές
• τον καφέ

Notice that the final -ς drops in the accusative singular.

Here it is definitely the noun coffee.

Why?

• it has the article τον
• it follows προτιμώ
• it is the thing being preferred

So τον καφέ means the coffee.

Greek also has καφέ as the color word brown, but that is not what is happening in this sentence.

Because απλό describes τον καφέ, so it must agree with καφέ in gender, number, and case.

Here καφέ is:

• masculine
• singular
• accusative

The adjective απλός changes like this:

• απλός = masculine nominative
• απλό = masculine accusative
• απλή = feminine
• απλό = neuter

So:

• πιο απλός would be the wrong case here
• πιο απλά is an adverb, meaning more simply, which is not what this sentence needs

Greek is using an adjective to describe the kind of coffee preferred: I prefer the coffee simpler / plainer.

Yes, in a way. But that is normal.

Πιο means more, and Greek often leaves the second half of the comparison unstated when it is obvious from context.

So πιο απλό here means something like:

• simpler
• more plain
• less fancy

The missing comparison is understood from the situation: compared with a cappuccino, or compared with coffee that has foam or extra additions.

This is similar to English sentences like I like my coffee simpler.

Greek word order is fairly flexible, and this sentence could be rearranged a little without changing the core meaning.

For example:

• Το αφρόγαλα στον καπουτσίνο είναι ωραίο, αλλά προτιμώ τον καφέ πιο απλό.
• Το αφρόγαλα στον καπουτσίνο είναι ωραίο, αλλά εγώ τον καφέ τον προτιμώ πιο απλό.

The version you have is natural. Its word order gives a slight emphasis to εγώ.

Also, a Greek speaker might choose slightly different wording depending on nuance, for example:

• ...αλλά εγώ προτιμώ τον καφέ σκέτο. = but I prefer coffee plain / black
• ...αλλά εγώ προτιμώ τον καφέ χωρίς αφρόγαλα. = but I prefer coffee without milk foam

So the original sentence is fine, but there are several natural ways to express the same general idea.