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；慎重的。careful作表语时，后面可以跟动词不定式、that从句。careful和about连用是“小心”的意思，后面通常接的是指物的名词、动名词或从句。

例句：
Careful on those stairs。走那几段楼梯要当心。
You have to be careful what you say on telly。在电视上说话时你必须很谨慎。
I asked her to be more careful in future。我要她以后更加小心。
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。