要不是你（过去）帮助了他,他可能已经失败了.
介词+宾语从句中的一种
except,but ,besides ,in
but that 要不是,若非
（1）I know nothing about my new neighbor except that he used to be a teacher
(2)I prefer his plans to yours in that I think his is more practical.(in that 表原因)
（3）He would have failed but that you helped him.