句子对错判断(如有错误，怎么样改)

判断句子对错，如有错误，怎么样改？

As we cant put our finger on what has been wrong, they have turned their back on us.

我和刘老师，English Expert两位老师的看法不同，这个句子是有语法错误的。错误比较隐蔽，大家非常难看出。问题出在turn their back on 这个习语上，这里back应该用复数backs, 由于主语是they。 在英语中有的习语中的名词单复数形式是不变的，如本句中的put our finger on， 另例：to turn the tables on sb. 在这个习语中，table 一直用复数。有的则需要依据主语的需要而发生变化。如句中的turn their back on... 例：Police officers turn their backs on President Obama.