名詞 alert:

1. condition of heightened watchfulness or preparation for action
   同義詞：qui vive
2. a warning serves to make you more alert to danger
   同義詞：alerting
3. an automatic signal (usually a sound) warning of danger
   同義詞：alarm, warning signal, alarum

動詞 alert:

1. warn or arouse to a sense of danger or call to a state of preparedness
   同義詞：alarm

形容詞 alert:

1. very attentive or observant
2. mentally responsive
3. not unconscious; especially having become conscious
   同義詞：awake
4. (usually followed by `to') showing acute awareness; mentally perceptive
   同義詞：alive, awake

1. Troops are on the alert against any possible aggression.
   軍隊(duì)戒備著可能的來犯。
2. The alert leopard went up to a deer quietly.
   警覺的豹子靜悄悄地走近一只鹿。
3. Although he's over eighty his mind is still remarkably alert.
   他雖已年過八十，但頭腦仍然十分機(jī)敏。
4. An alert listener will have noticed the error.
   耳朵尖的人能聽出這個錯。