1.be careful of表示“珍重，保重，注意，关注，留心，当心”，含有警惕、预防之意，后可跟人或物。
Be careful of the traffic.
注意交通安全。
Be careful of yourself! .
保重保重。
Be careful of your health. 保重身体。
Be careful of the man. 当心那人。
They had to be careful of floating chunks of ice.
他们必须当心大块浮冰。
提示
后跟where、how引导的从句或不定式时，要省去of。
Be careful where you take your date.
带约会对象去什么地方，一定要谨慎。
You must be careful how to spend the money. 你应当注意怎样花这笔钱。
They were careful to observe the proprieties.
他们恪守规矩。
He was ever careful to check his scripts.
他一向都认真检查剧本。
She had been careful to sweeten the deal with a rather generous cash payment.
她一直甚为审慎，想要通过支付大量现金促成这笔交易。
He is careful to show favour to no one.
他小心谨慎地不偏袒任何人。
Be careful to avoid being subjective and one-sided.
切忌主观片面。
2.be careful with表示“仔细，小心”，“小心处理/对待某事或物，对某事节约/节俭”指提醒某人用心、当心，后跟物作宾语，不可跟人。
Be careful with that ─ it cost a lot of money.
小心别弄坏那东西，它可值钱了。
Please be careful with my glasses.
请当心别打碎我的眼镜。
Be careful with the glasses.
小心这些玻璃杯。
It would force industries to be more careful with natural resources.
这将迫使企业更加节约资源。
You will have to make a special effort to train your child to be careful with her pocket-money.
你得格外注意培养孩子的节俭意识，零花钱要省着花。
Do be careful with the experiment you're doing.
在实验中切不可粗心大意。
We're newcomers here and must be careful with what we do.
我们新来乍到，得注意自己的一举一动。
be careful with the/your money花钱精打细算；吝啬、小气
We'll have to be careful with money but that doesn't mean we can't enjoy ourselves.
我们必须精打细算，可也并不是说我们就不能过得快乐。
The waiter was being very careful with the soap so as not to spill it.
服务生拿汤的时候很小心，怕把汤泼出来。
You must be careful with your homework. 做作业时你必须非常用心。（=be careful in doing…）
One cannot be too careful in guarding Party and state secrets.
保守党和国家的机密，慎之又慎。《汉英大词典》
You cannot be too careful in driving a car.
开车时再怎么小心也不为过。
3.be careful for表示“关怀，惦记，挂念”，后可跟人或物作宾语。
She is careful for her mother only. 她只惦记着她的母亲。
I am always careful for his progress in studies. 我经常关心他学业上的进展。
4.be careful about表示“谨慎，当心，注意”，“讲究，小心，当心；慎重的，注意...的；对...小心”。
People should be careful about their possessions and not leave them lying around
人们应该小心自己的财物，不要随便乱放。
He really must be careful about thinking aloud. Who knew what he might say?
他的确该注意自己说话不经思索的问题。谁知道他会说些什么？
Be extra careful about personal hygiene.
要非常注意个人卫生。
Be careful about your body language.
注意你的肢体语言。
Be careful about what you say. 说话要谨慎。
It's the one thing you've got to be careful about. 这件事情务必当心。
be careful about doing sth 慎重、小心、当心做某事
It is important, I think, for everyone to be careful about claiming victory.
我想，每个人宣布胜利时都应该慎重。
I think you should be careful about talking of the rebels as heroes.
我觉得你说话应该小心，不能把反叛分子当英雄。
最后，be careful有很多同义替换，如take care、look out、be attentive、be cautious，这些短语（句）也可以表达小心、当心、注意的意思。