Learn German: Der Käufer ist nervös, bevor die Auktion anfängt. - The buyer is nervous before the auction begins.

bevor is a conjunction used to connect clauses (it introduces the action die Auktion anfängt). vor is a preposition used before nouns, like in vor der Auktion (before the auction). Because this part of the sentence introduces a whole new clause with a subject and verb, you must use bevor.

Why is the verb anfängt at the very end of the sentence?

The word bevor is a subordinating conjunction. Words like this create a dependent clause, which forces the conjugated verb (here, anfängt) to the very end of that clause.

anfangen is a separable verb. Why isn't it split up here?

In a normal main clause, a separable verb splits, sending the prefix to the end (like Die Auktion fängt an). However, because bevor pushes the conjugated verb to the end of the clause, it bumps right into its own prefix. When this happens, they recombine into a single word: anfängt.

Is there a rule for knowing the gender of words like Käufer?

Yes! Nouns describing people that are created by taking a verb (like kaufen) and adding the suffix -er are grammatically masculine, so it is der Käufer. If it were specifically a female buyer, you would add -in to make it die Käuferin.