warn的用法：
“warn sb. of/about sth”意思是“针对…而警告某人”;“warn sb (not) to do sth”意思是“告戒某人(不)要做某事”;“warn sb. + that从句”意思是“警告某人说……”。如：They warned the passengers of thieves. (他警告路人小心窃贼) / Iwarn you that you will fail in the coming exams if you are still so lazy. (我警告你：如果你还这么懒在即将来到的考试中你会不及格的。) / He waswarned not to go out in the late night. (他受到警告不要在深夜出去)