副词是一种用来修饰动词,形容词,全句的词,说明时间,地点,程度,方式等概念.副词在句中可作状语,表语,短语.

careful是形容词，修饰名词。例如She is.a careful girl，她是个细心的女孩！carefully是副词，修饰动词。
You should do the homework carefully.你要认真做家庭作业。前者是形容词,作定语或表语.如：He is a careful boy.He is careful.后者是副词,作状语,修饰动词或整个句子.He reads the book carefully.