warn: may range in meaning from simple notification of something to be watched for or guarded against to admonition or threats of violence or reprisal: the weather bureau ~ed coastal areas to prepare for a hurricane.          guard单独使用表示守卫、警惕有力的保护; 和against联用表示防范提防


forewarn: stresses timeliness and regularly implies warning in advance of a foreseen risk or danger: they had been ~ed of the danger, and were prepared when it arose.


caution: stresses giving advice that puts one on guard or suggests precautions against either a prospective or present risk or peril: the doctor ~ed him against overeating.       (advice个人建议)


warn警告: 使用范围广，从简单提醒注意到警告或威胁报复，forewarn提前警告: 强调及时、提前警告预见到的风险危险，caution提醒小心: 提醒人防范已知或可能的风险危险


记忆方法:  1) 首字母WFC勿犯错<==警告

                2) 警告的意思是告知可能的危险风险mean to let one know of approaching or possible danger or risk.首字母WFC勿犯错<==警告