1. 后接介词 of 或 about, 表示“当心”“留意”等。如：

Be careful of the dog; it sometimes bites people. 当心那条狗，它有时候咬人。

You must be careful of [about] your pronunciation. 你要注意你的发音。

be careful 后一般不接介词 for。如：

我希望你要多注意身体。

正：I hope you’ll be more careful of [about] your health.

误：I hope you’ll be more careful for your health.

2. 表示做某事时要小心或仔细，其后通常接 in doing sth。如：

Be careful in crossing the street. 过马路时小心。

You cannot be too careful in driving a car. 开车时再怎么小心也不为过。

如果后接名词，则通常用介词with。如：

Do be careful with that vase! 务必小心那个花瓶!

You have to be careful with people you don’t know. 对不认识的人得当心。

3. 后接不定式，表示注意要做某事。如：

Be careful not to drop it. 小心不要把它掉了。

You must be careful to avoid being caught. 你必须小心，不要被抓住。

注意该用法与后接in doing sth的区别。比较：

He’s careful to read every sentence. 他注意读(即不漏读)每一个句子。

He’s careful in reading every sentence. 他读每个句子都很仔细。

4. 其后可接从句作宾语。如：

Be careful how you hold it. 小心拿好。

Do be careful (about) what you say. 说话一定要小心。

后接 what 从句时，可用介词 of, about 等，但通常省略，但是若是接 that, how, when, where 等引导的从句，则一般不用介词;另外，从句谓语若指将来，通常要用一般现在时代替将来时。如：

Be careful that you don’t hurt her feelings. (=Be careful not to hurt her feelings). 当心别伤害她的感情。