What is the meaning of "la'Sheker mi'Geva'os Hamon Harim"?
Rashi: We served idolatry [on numerous mountains].
Radak #1: We served idolatry in vain - it did not help us. We thought to be saved due to our Avodah on the hills; really, Yisrael's salvation is only from Hashem. "Hamon" has a Kamatz under the Hei, for it is not Samuch to Harim. If it were Samuch, it would have Chataf Patach, like "Av Hamon Goyim" (Bereishis 17:5). Also the masses who gathered on the mountains to serve idolatry, was in vain. The prefix Beis should precede "Harim"; it is omitted, like "ha'Nimtza Veis Hashem" (Melachim II, 12:11), "Liznos Beis Aviha" (Devarim 22:21) and similar verses.
Radak citing Targum Yonasan: In vain we served [idolatry] in high places. We did not benefit from roaring on mountains.
Radak #2: We expected help from the hills (nations), and from the mountains, i.e. many nations in the north; it was in vain - "Hevel b'Tzipiyasenu Tzipinu El Goy Lo Yoshi'a" (Eichah 4:17). So Chazal expounded.
Malbim: A voice comes between the mountains, and an echo from the hills. One who is in the valley does not know which is the true voice, and which is the echo (Sheker); the false Nevi'im tell us to do idolatry.
What is the meaning of "Achen"?
Rashi: In truth.
Why does it say "ba'Shem Elokeinu Teshu'as Yisrael"?
Radak: Hashem is favorably inclined to help Yisrael when they go in His ways, and He always guarded them - unlike other nations.
Malbim: We are too weak to repent. The salvation of Yisrael is possible only via Hashem's help.
