quick to see, understand, and act in a particular situation

I'm not feeling very alert today - not enough sleep last night! 今天我感觉特迟钝——昨晚没睡好！

A couple of alert readers posted comments on the website pointing out the mistake. 有几个眼尖的读者在网上发表评论，指出了这个错误。

Parents should be alert to sudden changes in children's behaviour. 父母应该对孩子行为的突然变化有所警觉。

a warning to people to be prepared to deal with something dangerous

a bomb alert 炸弹警报

The army was put on (full) alert as the peace talks began to fail. 当和谈开始破裂的时候，军队进入（全面）戒备状态。

The public were warned to be on the alert for (= watching carefully for) suspicious packages. 有关部门提醒公众，要警惕可疑包裹。

to warn someone of a possibly dangerous situation

An anonymous letter alerted police to the possibility of a terrorist attack at the airport. 有人给警察写了一封匿名信，警告机场可能会受到恐怖袭击。