How were Yisrael shown that Hashem alone is Elokim?
Rashi (citing the Pesikta): When Hashem gave Yisrael the Torah, He opened the seven Heavens and they saw that He was alone (that there were no other beings there beside Him). 1
Targum Yonasan, Hadar Zekenim, Seforno: It refers to the signs and miracles mentioned in the previous Pasuk. 2
What is the consequence of knowing that Hashem is Elokim?
Hadar Zekenim: You should realize that one cannot escape Him. He can kill or bring to life
Why does it say here and elsewhere "Hashem Hu ha'Elokim", and it never says "ha'Elokim Hu Hashem"?
Rashi: Refer to 6:4:3:1*.
R. Bachye: There are open miracles (associated with the name Hashem) and hidden miracles (amidst nature, which is associated with the name Elokim; its Gematriya is 86, like 'ha'Teva'). The hidden are included in the open. One with intellect will understand this.