going back and forth：為一固定用法,意思是“來來回回”；加上定冠詞 就是一個“名詞短語”,要作為一個獨立的名詞看待.即：the going-back-and-forth .
We knew（that） the going-back-and-forth wasn't going to cut it any more.
中間省略了that ,后面是賓語從句,the going-back-and-forth為賓語從句的主語.
cut：在這里為：“與…相交；與…相切；穿梭”的意思.
We knew the going back and forth wasn't going to cut it any more.
“我們知道：不能再這樣來來回回繼續(xù)穿梭下去了.”
直譯就是：
我們知道：這種“來來回回”再不要繼續(xù)穿過來穿過去了.
it =the going-back-and-forth
明白了嗎?