Questions & Answers about Vem är den tredje studenten?
Why are we using both den and studenten? Doesn't studenten already mean "the student"?
Yes, studenten does mean "the student". However, Swedish uses a rule often called double definiteness. When you put a describing word or ordinal number (like tredje) in front of a definite noun, you must also add a freestanding "the" word. For en-words like student, that word is den. So you are literally saying "the third the-student".
Could I ask Vilken är den tredje studenten? instead?
You could, but the meaning shifts slightly. Vem means "who" and asks for the person's identity or name. Vilken means "which" and would sound like you are asking someone to point out which person in a physical group is the third one.
Adjectives usually take an -a ending when used with den. Why does tredje end in -e?
Tredje is an ordinal number, meaning "third", rather than a standard adjective. In Swedish, ordinal numbers from "third" and above (like fjärde for fourth, femte for fifth) naturally end in -e. They are fixed and never change their endings to match the noun.
Does studenten change if the student is female?