1. When Amy broke up with her boyfriend Ben started courting her.2. Nation A attacked Nation B when Nation B's army is weak. The attack caught Nation B off guard. or The attack caught Nation B by surprise.
 take advantage of somebody when they are weak or in a disadvantageous situation catch somebody by surprise catch somebody off guard英文中沒有此成語的說法，所以只能按情況白話翻譯。Catch somebody by surprise 跟 Catch somebody off guard 意思是當某人沒注意到的時候做了某件事情
The company's CEO embezzled and drained all the company's money. What a despicable lowife.