When will Yehoshua build the Mizbe'ach?
Rashi: The verse is out of order. He built it the day that Yisrael crossed the Yarden. Radak
Malbim: Why did the Torah repeat the Mitzvos (a) to erect rocks, (b) to plaster them and (c) to write the Torah on them (Devarim 27:2-4,8)? And why did it change from "b'Yom she Ta'avru ha'Yarden" to "b'Avrechem"? The first time was in the middle of crossing the Yarden. In this merit, they would enter the land. Days later, they erected the same rocks in Har Eival for all generations, plastered over the first writing, and wrote again on them a professional writing.
QUESTIONS ON RASHI
Rashi writes that he built it the day that they crossed the Yarden. Why does it say "Yivneh" (future)?
Radak: (They offered on the Mizbe'ach.) This teaches that a Bamah is permitted only through a Navi. 1 Also elsewhere, it says "Az" and the future tense for something that occurred, e.g. "Az Yashir Moshe."
Bamos were permitted when the Mishkan was in Gilgal, Nov and Giv'on (Zevachim 14:5,7). Why must a Navi permit it?! Perhaps he means that every time there was a change of status, one could not offer on a Bamah until a Navi permitted it. Do not say that we expound the periods of Heter from "Ki Lo Vasem