Realize means "to grasp clearly," "to understand fully." It is a stronger word than know, which also means "to perceive" or "to apprehend" but without the thoroughness and completeness suggested by realize. Realize also suggests the idea of warning or emphasizing."I know that you are tired." "Do you realize that you might catch a cold in your weakened condition?" "You know that you are already late, but do you realize what the penalties for lateness may be?"
Dictionary of problem words and expressions. Harry Shaw. 1975.