形容詞 on guard:

1. vigilant
   同義詞：on one's guard, upon one's guard, on your guard

1. The citizens were warned to be on guard against a sudden night attack.
   市民們得到通知，要提防夜間的突然襲擊。
2. Be on guard when somebody flatters you.
   有人奉承你時(shí)，一定要當(dāng)心。
3. There are soldiers on guard at the gate.
   大門口有士兵在守衛(wèi)。
4. While danger threatens we must all be on guard.
   危險(xiǎn)正逼近，我們必須保持警惕。