1.under the sun
这个很容易被大家理解成“在太阳底下”，其实不然，真正意思跟太阳没有半点关系。是指“（强调事物数量很大）天下，世上，全世界 ”，还有“究竟；到底 ”的意思。
例句：We sat there for hours talking about everything under the sun
我们在那儿坐了几个小时，谈天说地。
We talked about everything under the sun.
天南地北，我们无所不谈。
天南地北，我们无所不谈
We sat there for hours talking about everything under the sun
我们在那儿坐了几个小时，谈天说地。
They had spent the previous night chatting about everything under the sun.
昨天晚上，她俩说东道西聊了一夜。
The fashion-conscious will go for anything under the sun!
流行意识将无所不在。
To a sophisticated person there is nothing new under the sun.; He who has sailed the
seven seas doesn't think much of mere rivers.
曾经沧海难为水。
《汉英大词典》
Where under the sun could I have put the purse?
我究竟把钱包放在哪儿了？
2. in the sun
那“在太阳下”应该怎么表达呢？其正确翻译应该是“in the sun”。
例句：I like looking for my shadow in the sun.
我喜欢在阳光下找自己的影子。
Don’t read in the sun.不要在阳光下看书。
We sat in the sun .
我们坐在阳光下。
另外，“in the sun”还有“阳光充足的地方”的意思：in a place where it is warm and the sun shines a lot
They've booked a holiday in the sun (= in a place where it is warm and the sun shines a lot) .
他们已经预订好了去阳光充足的地方度假。
Dena took them into the courtyard to sit in the sun
德娜带他们到院子里晒太阳。
