Breakdown of Die Schauspielerin posiert mit dem Designer, einem bekannten Künstler.
ein
a, an (eine)
mit
with
der Künstler
the artist
die Schauspielerin
the actress
Questions & Answers about Die Schauspielerin posiert mit dem Designer, einem bekannten Künstler.
Why is it einem bekannten Künstler instead of the nominative ein bekannter Künstler?
This is a noun apposition, which is an extra piece of information placed right next to another noun to describe it. In German, an apposition must always match the case of the noun it describes. Because dem Designer is in the dative case, the apposition describing him must also be in the dative case (einem bekannten Künstler).
Why does the adjective bekannten end in -en?
Adjective endings depend on the case, gender, and the article used before them. Here, we are in the dative case with a masculine noun (Künstler) following the indefinite article einem. In the dative case, adjectives following an article always take the -en ending.
Just to review, what exactly is forcing dem Designer into the dative case to begin with?
The preposition mit (with). It is a strict dative preposition. Whenever you use mit, the noun phrase immediately following it must be in the dative case, which is why we see dem Designer instead of the nominative der Designer.
Do I always need a comma before an apposition like einem bekannten Künstler?
Yes. In German, an apposition that adds extra descriptive information is separated from the rest of the sentence by commas. If the sentence continued after Künstler, you would need another comma there, too.