bevor or vorher?
b1bevor is a conjunction and needs a full clause with the verb at the end, vorher is an adverb standing on its own in a main clause. The same pair exists for the other direction: nachdem and danach.
bevor
Introduces a subordinate clause: bevor plus subject, verb at the end. Both parts are usually in the same tense.
Bevor ich gehe, räume ich auf.Before I leave, I tidy up.
vorher
Refers back to something already said and stands in its own main clause, usually in position 1.
Ich gehe gleich. Vorher räume ich noch auf.I'm leaving shortly. Before that I'll tidy up.
The mistake
Vorher ich gehe, räume ich auf.
Bevor ich gehe, räume ich auf.
„vorher“ cannot introduce a subordinate clause. A clause with its own subject needs the conjunction „bevor“.
Rule of thumb
Rule of thumb: conjunction plus clause (bevor, nachdem), adverb after a full stop (vorher, danach).
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