Besides if, there are other possible translations for אִם, depending on the context. But it is very true that, as Cowles says, oaths are often elliptical in this way. If not [אִם־לֹא], is often translated as surely. Keith also agrees with Cowles construction.
 The Cambridge Paragraph Bible: of the Authorized English Version (1873). (Heb 3:11). Cambridge: Cambridge University Press.
 The Cambridge Paragraph Bible (Ps 95:11).
 Vincent Word Studies. Studylight.
 Heinrich Meyer's Critical and Exegetical Commentary on the New Testament (1832). Studylight.
 Johann Albrecht Bengel’s Gnomon of the New Testament (1897). Studylight.
 Matthew Poole’s English Annotations on the Holy Bible (1685). Studylight.
 F.W. Farrar. Cambridge Greek Testament for Schools and Colleges (1896). Studylight.
 Whedon, Daniel. Whedon’s Commentary on the Bible (1874-1909). Studylight.
 Bullinger, E. W. (1898). Figures of speech used in the Bible. London; New York: Eyre & Spottiswoode; E. & J. B. Young & Co.
 “Ellipsis may be found in protases as well as in apodoses” say Joüon, P., & Muraoka, T. (2006) in, A grammar of biblical Hebrew (p. 594). Roma: Pontificio Istituto Biblico.