warning/ 'wɔːnɪŋ /

1.noun [c] 
(action,words,sign) 警告

2.noun [c/u] 
(notice) 预兆

to give sb a warning
给某人一个警告
let this be a warning to you
让这事成为你的鉴诫
without (any) warning
出人意料地
3.noun
(without notice) 毫无征兆地