Why does the Torah write "Tih'yeh Laylah bo" - in the future?
Oznayim la'Torah #1: Hashem was speaking to Yisrael in the daytime, so he added that, when night arrived, a pillar of fire would cover the camp.
Oznayim la'Torah: It is a hint that even in the night of Galus there will be a pillar of fire - in the form of the Beis-Hamikdash that will descend from Heaven - as Rashi and Tosfos write in Sukah, 41a. 1
Oznayim la'Torah: And as is hinted in Zecharyah 2:9.
Since the Cloud and the Fire covered the Mishkan when they encamped, why does the Torah write "be'Chol Mas'eihem"?
Rashi: Because the location where they encamped was also called 'Masa'. 1
As we find in Lech L'cha Bereishis, 13:3, and in Mas'ei Bamidbar, 33:1 (Rashi).
