I was only joking; who would have thought he would take it seriously.

我是开玩笑，谁知他竟当真了。

He is joking. Don't take it seriously.

他是跟你开玩笑呢。你别当真。

He was joking. Don't take it seriously.

他是跟你闹着玩儿的，你别当真。

Don't take it seriously.

不要认真吗！

Don't take it seriously. It's only a joke.

不要认真嘛！这不过是开个玩笑而已。