What is the meaning of "Achas Nishbati b'Kodshi"?
Radak: The Shevu'ah is one; it will not change. I swore b'Kodshi - in My Kedushah.
Malbim: One who swears to his friend conditionally, there are two matters - the stipulation and the Shevu'ah. The Shevu'ah is obligatory only if the stipulation is fulfilled, e.g. if they will go in the way of Toras Hashem, He will fulfill the Shevu'ah for them. One who swears without a stipulation, there is only one matter - the Shevu'ah. It is as if I swore without a stipulation.
Why does it say "Im"?
Radak: This is an expression of a Shevu'ah - if I will do so and so
What is the meaning of "Im l'David Achazev"?
Rashi: This is like "Lo Yechazevu Meimav" (Yeshayah 58:11) (will not cease).
Malbim: The Shevu'ah will not cease in any case.