In what forms of training do you participate [in/to]?
Should I end that sentence with "in" or "to"? If I add "in" at the end, should I cut the "in" at the beginning?
Better to keep it at the beginning and forget about it at the end. People will argue with this, but I still think it sounds better not to end a sentence on a preposition if it can be avoided.
It is to participate in sth. But when you begin a question with "In what forms of training do you participate? - then your sentence is complete. A further preposition at the end of the sentence is simply wrong.