Honestly, in my opinion neither of these sentences sound natural to me. I would simply say, "I truly want to pass the exam" or "I really want to pass the exam." " Below are corrections to your sentences: 1.) Heaven forbid that I fail (it should be 'fail' not fell) the exam. 2.) Heaven forbid that I pass the exam. I would not use this because the phrase 'Heaven forbid' tends to be used when you are expressing that something does NOT happen and this is not case. You want to pass the exam, so I would definitely not use the phrase heaven forbid. I hope my explanation makes sense and let me know if you have questions. Good luck on the exam! :)

Heaven forbid (that) I fail (don't pass) the exam! heaven forbid (also God forbid) The expression 'Heaven forbid' is a way of saying that you hope something does not happen. Example: Heaven forbid (that) the business should ever close down.

1, Heaven forbid that I fell the exam.