hardly when ＝刚一...就...
举几个例句给你就容易理解用法：
Hardly had the foot ball game begun when it started raining.
足球比赛才刚刚开始就下起雨来.
Hardly have I get home when the telephone rings.
我一到家电话铃就响了.
另外,hardly 也可以以 scarcely 来替代.
例如：
Hardly/Scarcely had we arrived,when it began to rain.
我们刚一到就下起雨来了