1、back to square one 短语出自a game board（棋盘游戏），玩法是从第一个格子（on the first square）玩起，所以if you go back to square one,that means you are going back to the beginning of something.

2、back to square one

back是副词，to是介词，square one是名词组，是第一个方格的意思，结构类似number one。所以介词组 to square one 修饰副词 back。

3、例句

We've got to get this done without going back to square one.我们必须在不从头开始的情况下完成这项工作。

The treatment failed and I was back to square one.治疗失败了，我又回到了起点。

4、知识延伸

back in time 回到从前、时光倒流

right back at you 彼此彼此、你也是

Let me back up 我倒车

back me up 帮我说句话

get back at you 报复你

take a back seat 退居二线

know you like the back of my hand 对你了如指掌

We go way back.我们认识很久了。

get off my back 别烦我