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。