Kannst du mir den Kollegen vorstellen?
the verb "vorstellen" has 2 "Objects": which is dative and which is Akkusative?
"jemanden (Akkusativ) jemandem (Dativ) vorstellen"
If Person A is the direct object (Akk) and Person B is the indirect object (Dat), then A is introduced to B. This means person B is the one who gets to know person A.
Kannst du ihn (Akk) mir (Dat) vorstellen.
Can you introduce him to me.
In your example, "mir" is Dative and "den Kollegen" is Akkusative.
If you want to find out the declination of a word, you can always ask the same questions:
- "Who or what does something?" if you want to find the Nominative (the protaginist of your sentence)
- "Whose?" if you want to find the Genitive (Possessive)
- "Whom? To whom?" if you're looking for the Dative
- "Who or what?" if you're looking for the Akkusative (the object)
In your sentence:
- "Who or what does something?" - du = Nominative
- "Whose?" - not present
- "Whom? To whom?" - mir = Dative
- "Who or what?" - "den Kollegen" = Akkusativ
I hope this helps you in the future. :)
"Mir" is dative (https://en.wikipedia.org/wiki/German_pronouns#Personal_pronouns). Kollege is masculine so takes "den" in the accusative.