John Gill Commentary Deuteronomy 4:9

"Only take heed to thyself, and keep thy soul diligently, lest thou forget the things which thine eyes saw, and lest they depart from thy heart all the days of thy life; but make them known unto thy children and thy children`s children;" — Deuteronomy 4:9 (ASV)

Only take heed to yourself
To walk according to this law, and not swerve from it:

and keep your soul diligently ;
from the transgressions and breaches of it:

lest you forget the things which your eyes have seen ;
either the statutes and judgments set before them, and the circumstances of the delivery of them; or the punishment inflicted on the breakers of them; or the favours bestowed on those that observed them:

and lest they depart from your heart all the days of your life ;
out of your mind and memory, and have no place in your affections, through a neglect and disuse of them:

but teach them your sons, and your sons' sons ;
their children and grandchildren, that they may be trained up in them in their youth, and so not depart from them when grown up, and in years; see (Deuteronomy 6:7) .