1. 表示某人做某事很粗心，通常用于it is careless of sb do sth.如：
It was careless of you to leave your camera in the taxi. 你把照相机留在计程车里实在太不小心了。
该句式也可换成以“人”作主语的句子。如：
你犯这样的错误真是粗心。
It was careless of you to make such a mistake.
=You were careless to make such a mistake.
注意，这样用时句首的it为形式主语，不能改为 that.但是这类句子后面不出现不定式，那么句首的it也可换为 that.如：
It was very careless of him.=That’s very careless of him. 他太粗心了。
2. 比较be careless to do sth与be careless in doing sth:前者中的不定式为结果状语，指由于粗心而做了某事；后者中的in doing sth表示时间，其中的in相当于while，指在做某事时很粗心。如：
He was careless to make the mistake. 他很粗心，结果出了错。
He was careless in typing the letter. 他打这封信时很粗心。
3. 表示“对……不在意或不关心”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。
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