Question
Why is the ring placed on the forefinger of the bride, on her right hand?
Answer
By accepting the ring the bride demonstrates her willingness to get married and she agrees to be her husband’s wife. Therefore she accepts with her dominant hand (right if she is right-handed) and with her strongest finger – the index finger.
Immediately after she has accepted the ring she may move it to her ring finger, since it now belongs to her, and she can do with it as she wishes. I hope this answer helps.
Rabbi David Sedley