to build castles in the air 白日做梦，空想

英语搭配“castles in the air 浮在云间的城堡”的含义是“几乎不可能实现的梦想或计划”，这与我们常说的汉语成语“空中楼阁”和“黄粱一梦”含义相近。所以，表达“to build castles in the air”被用来形容人的幻想不切实际，可以理解为“做白日梦”的意思。

例句

Pat is such a dreamer. She's always building castles in the air.

帕特真是个梦想家。她总是梦想那些无法实现的计划。

My mum told me to stop building castles in the air and find a real job, but there's no way I'm giving up on my dreams of becoming a professional footballer.

我妈告诉我不要一天到晚总做白日梦，该去找一份真正的工作了，但我是绝对不会放弃成为一名职业足球运动员的梦想的。

Ding needs a proper business plan if she is serious about setting up her own shop! At the moment she's just building castles in the air.

如果小丁在认真考虑创建自己的店铺的话，那她就需要准备一份像样的商业计划。目前她只是在空想而已。