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。
注：be careful 后一般不接介词 for。如：
正：I hope you’ll be more careful of[about] your health.
误：I hope you’ll be more careful for your health.
2. 后接介词 with(其后多接名词)或 in(其后多接动名词)，表示做某事时很小心或仔细。如：
He’s careful with [＝in doing] his work. 他工作很仔细。
Be careful with the vase; it is valuable. 这花瓶很贵，小心点。
Be careful in dealing with such people. 和这种人打交道要小心。
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。
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。
Be careful how you hold it. 小心拿好。
Do be careful what you say. 说话一定要小心。
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。
注：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( 见上例)。
正：It was careless of you to leave your wallet here.
正：You were careless to leave your wallet here.
以上第一句句首的 it 为形式主语，不能改为 that。但是若以上句型后面不出现不定式，那么句首的 it 也可换为 that。如：
正:It was very careless of him.
正:That’s very careless of him.
正：He was careless to make the mistake. 他很粗心，结果出 了错。(句中的不定式为结果状语)
正：He was so careless as to make the mistake. (同上)
正：He was careless in typing the letter. 他打这封信时很 粗心。(句中的 in typing the letter有类似 when he was typing the letter 的意思)
误：He was careless to type the letter. (若此句视为正句，则只能理解为：他很粗心，结果把那封信打印了出来)
3. 表示“不在意的”、“不关心的”等，其后可接介词 about, of, with 等。如：
He’s careless of [about] his speech. 他讲话随便。
She is careless of [with] his health. 她不注意身体。
He is careless with his pronunciation. 他对语音不注意。
Don’t be so careless with your money. 你花钱不要这样随便。