这是the more...the more 句式,：越.越.”如
The more careful （形容词）you are,the fewer mistakes you will make.你越小心,你犯的错误越少.
The sooner,the better.越快越好.
The more progress you make,the happier you will be.
上面都是形容词的比较级,还可以是副词的.The more carefully you drive,the safer you are.
你开车越小心,你越安全.
所以,本句中第一个横线前的副词carefully 是修饰动词read 的,而 横线的careful 是做you are careful 的表语的.