1：no soon...than和hardly...when的区别2：regret to do 和regret doing的区别

no sooner…than

它的意思相当于“刚做完A这件事，就做B这件事”。例如：

He had no sooner returned than he bought a house.
他一回来就买了一套房子。

如果no sooner 位于句首，主句要用倒装结构。例如：

No sooner had he arrived than he went away again.
他刚到就又走了。

No sooner had the game started than it began to rain heavily.
运动会刚开始，天就下起了大雨。

有一点要注意，no sooner…than 一般用来描述做过的事情，它不能用于表示将来的事。

hardly…when...

它的意思是“几乎未来得及做完A这件事，紧接着就开始B这件事。”
使用这个短语时，一般用于过去完成时；并且when有时可改用before。例如：

He had hardly finished the article when the light went out.
他刚写完文，灯就熄了。

He had hardly had time to settle down when he sold the house.
他还未安顿下来，就卖掉了那所房子。

另外，如果把hardly放于句首，主句要用倒装结构即谓语动词要提前。如：

Hardly had I finished eating when he came in.
我刚吃完他就进来了。

Hardly had she gone out when a student came to visit her.
她还没走出家门，就有一个学生来看望她。

regret to do 后悔或遗憾没做过的事,
I regret to tell you