careful，carefully都是小心的意思，前者是形容词,作定语或表语。后者是副词,作状语,修饰动词或整个句子。

例句：

1、He is a slippery customer, and should be carefully watched.

他是一个狡猾的顾客，要小心提防

2、Be careful of the traffic.

注意交通安全。

3、Please be careful with my glasses (= Don't break them) .

请当心别打碎我的眼镜。