C. F. Keil and Franz Delitzsch on 1 Samuel 20:3

1Sa 20:3

But David had looked deeper into Saul’s heart. He replied with an oath (“he sware again,” i.e., a second time), “Thy father knoweth that I have found favour in thine eyes (i.e., that thou art attached to me); and thinketh Jonathan shall not know this, lest he be grieved. But truly, as surely as Jehovah liveth, and thy soul liveth, there is hardly a step (lit. about a step) between me and death.” כּי introduces the substance of the oath, as in 1Sa 14:44, etc.