Why are the K'lalos here written in the third person, whereas those in Vayikra are written in the first person? And why is copper ascribed to Shamayim and iron to Aretz, in the reverse order in which they re written in Bechukosai Vayikra, 26:19?
Rashi: Moshe himself said these K'lalos (Refer to 28:15:1:1), and he presented them in a more lenient form than Hakadosh-Baruch-Hu, who said those in Vayikra in the plural 1 , and ascribed iron, which does not sweat (produce moisture), to the heaven, and copper, which sweats, to the earth. Whereas, based on Moshe's curse, the heavens will produce a little rain, and the earth will not sweat to spoil the few fruits that grow. 2
Targum Yonasan: 'The heaven ... will be dry like copper which sweats so tht the dew and rain will not suffice for you; and the earth ... will be like iron whih does not sweat, and it will not moisten the trees, the spices, the the choice fruits and the vegetables.
Why does the Torah write "Shamecha asher al Roshcha ... "?
Oznayim la'Torah: Because, since the B'rachos and the K'lalos were said in the singular, 1 it is possble that the heaven above Re'uven might open and send rain or close and withhold it, for Reuven but not for his neighbor Shimon.
Refer to 28:2:0.1:1.