1. miss the point
It sounds like someone does not understand something, and that's exactly what "miss the point" means.
I'm not sure. Can you explain it again? I think I missed the point.
2.In your dreams
"In your dreams" is a different way of saying, "I don't believe you" or "there's no way that's true."
A: Guess what? This weekend, I caught a fish this big.
B: No way.
A: Yeah, It was huge!
B: In your dreams! There's no way you caught a fish that big.
3.A big deal
"An 18-year old boy has got into graduate school, that's a big deal.
4. Go-ahead，两个词当中加联接号，go-ahead就成了名词，意思是批准或下令
I've already given them the go-ahead, what are they waiting for.
go ahead当中没有连接词时用作动词
go ahead, do your homework
5. Mercy这个词是指仁慈，但是at the mercy of... 是指受某人的支配或控制
It's painful to live at the mercy of someone else
你周末想去爬山吗？We are at the mercy of the weather - 这在于天气