Why does it say "Zichru l'Olam Briso"? In Tehilim 105:8, it says "Zachar"!
Radak (here and Tehilim 105:8): There it is like "Asher Karas Es Avraham" (verse 16) - that Bris He remembers for Avraham's seed forever. The land is always of Yisrael, even when they are exiled from it; they will return to it. Here it commands Yisrael to always remember the Bris.
What is the meaning of "Davar Tzivah l'Elef Dor"?
'Rashi', Rashi (Tehilim 105:8) #1: The Torah, which He commanded to inform them in the world after 1000 generations, and He saw that the world does not exist without Torah, and He removed 974 (and gave it after only 26).
Rashi (Tehilim 105:8) #2: He remembered for Yisrael His Bris that He commanded, and promised to guard it for them for 1000 generations ? "Shomer ha'Bris veha'Chesed l'Ohavav ul'Shomerei Mitzvosav l'Elef Dor" (Devarim 7:9).
Radak #1: What He promised to Avraham, He commanded that it last 1000 generations.
Radak #2: He decreed; this is like "Hashem Lo Tzivah" (Eichah 3:37). What He decreed Bein ha'Besarim, to give to Eretz Kena'an to their children, will be for 1000 generations.
Radak (Tehilim 105:8, citing Shocher Tov): We expound l'Aluf b'Dor, i.e. Avraham Avinu; Elef is like 'Taku'a Alfa l'Shemen' (Menachos 8:3; it is best).
Malbim: Refer to 16:8:3:3.
Why does the verse switch from "l'Olam" to "l'Elef Dor"?
Radak: The Bris He remembers for Avraham's seed forever. The promise to Avraham [to give the land to his children] was for 1000 generations.
Radak (Tehilim 105:8): There is no difference ("l'Elef Dor" is not precise).
Malbim (Tehilim 105:8): The Bris with the Avos does not depend on people. "L'Olam" refers to time that is not divided. What he commanded to people, is for generations ? a generation comes and goes. The coming verses explain the Bris and command.
