The Hebrew particle pen is a warning conjunction meaning lest, beware that not, take care that not. It introduces subordinate clauses expressing danger or negative consequences to be avoided. In English it corresponds to 'so that... not' or 'for fear that.'
The frequency of pen in the Law reveals God's pastoral heart — over and over, the commands come with protective warnings: lest you forget, lest you serve other gods, lest your heart be lifted in pride, lest your children be led astray (Deuteronomy 4:9; 6:12; 8:11–14). The word captures the loving insistence of a father warning his child from the edge of a cliff. Proverbs uses it repeatedly: guard your heart, lest you stumble. The New Testament echoes it in warnings about falling away (Hebrews 2:1; 3:12-13). Vigilance, not fear, is the proper response to pen.