What is the birth of mountains?
Rashi: It is their creation. The same applies to "va'Techolel Eretz v'Sevel."
Malbim: It is the birth of what sprouts from them (vegetation). Radak: The foundations are born via Hashem's command.
Why does it mention only the land, but not Shamayim?
Radak: It wants to discuss man, who stands a short time in the world. An individual lives in the land a short time, even though the species remains. In Shamayim, also the individuals remain. It mentions mountains, for they last longer than other parts of the land.
What is the difference between "Eretz" and "Tevel"?
Radak: Tevel is what is settled; Eretz is even what is not settled. When it is mentioned with Tevel, it is only what is not settled.
What is the meaning of "ume'Olam Ad Olam"?
Rashi: From the first world until the last.
Radak: "Me'Olam" is before the world was created. The same applies to "me'Olam Nisachti" (Mishlei 8:23). "Ad Olam" is the end of the world.
What is the meaning of "Atah Kel"?
Radak: You have [all] ability.
Malbim: You are the source from which Ruchos came, and they will return to You. You are their dwelling place forever.