tn The plural “my backs” is according to Gesenius an extension plural (compare “face,” a dual in Hebrew). The word denotes a locality in general, but that is composed of numerous parts (see GKC 397 §124.b). W. C. Kaiser says that since God is a spirit, the meaning of this word could just as easily be rendered “after effects” of his presence (“Exodus,” EBC 2:484). As S. R. Driver says, though, while this may indicate just the “afterglow” that he leaves behind him, it was enough to suggest what the full brilliancy of his presence must be (Exodus, 363; see also Job 26:14).
tn The Niphal imperfect could simply be rendered “will not be seen,” but given the emphasis of the preceding verses, it is more binding than that, and so a negated obligatory imperfect fits better: “it must not be seen.” It would also be possible to render it with a potential imperfect tense: “it cannot be seen.”