riddle
谜，谜语 

例句：
That painting is still a riddle to us.
那幅画对我们来说仍然是个谜。

play it by ear
随机应变
 
例句：
Facing his enemy, he had to play it by ear.
面对敌人，他只有随机应变了。

Thank you for having us.
谢谢你请我们做客。 

对话：
A: I'm afraid I've got to go now. Thank you for having us. We had a good time.
B: It was a pleasure.
　
A：恐怕现在得走了，谢谢你邀请我们做客，我们玩得很开心。
B：你们玩得好，我也很高兴。