careful的比较级是more careful,最高级是the most careful。careful的意思是小心;注意;谨慎;细致的;精心的;慎重的，carefully 是它的副词形式。
例句：
Can't you be a bit more careful with my bike？你骑我的车就不能更小心点吗？
Such an occasion demands the most careful consideration。这样的场合需要最周到的考虑。
We all think he is one of the most careful students in his class。我们都认为他是班里最细心的学生之一
careful:
比较级:more careful
最高级是:the most careful
careful变为carefully,是因为careful是形容词，通常形式名词，不能修饰动词、形容词、副词。当需要修饰动词、形容词、副词时，就需要用副词，把形容词careful变为副词就是直接在形容词careful的后面加后缀ly变为carefully。例句：
1.The students are listening to their teacher carefully.
同学们正在仔细地听老师讲课。
副词carefully修饰动词listen.
