Drivers must be on the alert for traffic signals.驾驶员必须密切注意交通信号.They were constantly on the alert not to be taken by surprise.他们一直处于常备不懈状态以防突然袭击.Hearing this, I was instantly on the alert.我一听这话, 当下就警惕起来....

警戒着， 随时准备着， 密切注意着...

on the alert的音标：on the alert的英式发音音标为：[ɔn ðə ə'lə:t]on the alert的美式发音音标为：[ɑn ði ə'lɚt]...