In the Vulgate Bible, I came across this sentence.
Vivit Dominus Deus Israel, in cuius conspectu sto, si erit annis his ros et pluvia, nisi iuxta oris mei verba.
[As] the Lord God of Israel lives, in whose sight I stand, there will not be dew and rain these years, unless according to the words of my mouth.
1 Kings 17:1
The translation makes sense, but I don't understand the si...nisi construction. I know the basics of conditionals, but I haven't seen these two words used in combination before. Could anyone explain? Also, where do we get the "not" adverb in the clause, "there will not be dew and rain"?