as. according as.
heaven. the heavens.
but. except it: i.e. until it. Note that the four succeeding tenses are pasts, and in the singular number. The waters do return, as stated in other scriptures (Psalms 135:7; Jeremiah 10:13; Jeremiah 51:16).
Related Commentaries of Isaiah 55:10
Isaiah 55:10
10 For as the rain cometh down, and the snow from heaven, and returneth not thither, but watereth the earth, and maketh it bring forth and bud, that it may give seed to the sower, and bread to the eater: