1. 后接介詞 of 或 about, 表示“當心”“留意”等。如：

Be careful of the dog; it sometimes bites people. 當心那條狗，它有時候咬人。

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

be careful 后一般不接介詞 for。如：

我希望你要多注意身體。

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

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

2. 表示做某事時要小心或仔細，其后通常接 in doing sth。如：

Be careful in crossing the street. 過馬路時小心。

You cannot be too careful in driving a car. 開車時再怎么小心也不為過。

如果后接名詞，則通常用介詞with。如：

Do be careful with that vase! 務必小心那個花瓶!

You have to be careful with people you don’t know. 對不認識的人得當心。

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. 你必須小心，不要被抓住。

注意該用法與后接in doing sth的區別。比較：

He’s careful to read every sentence. 他注意讀(即不漏讀)每一個句子。

He’s careful in reading every sentence. 他讀每個句子都很仔細。

4. 其后可接從句作賓語。如：

Be careful how you hold it. 小心拿好。

Do be careful (about) what you say. 說話一定要小心。

后接 what 從句時，可用介詞 of, about 等，但通常省略，但是若是接 that, how, when, where 等引導的從句，則一般不用介詞;另外，從句謂語若指將來，通常要用一般現在時代替將來時。如：

Be careful that you don’t hurt her feelings. (=Be careful not to hurt her feelings). 當心別傷害她的感情。