absent-mined
心不在焉的

例句：
The absent-minded professor put salt in his coffee and sugar on his egg.
那位心不在焉的教授把盐放进了咖啡而把糖加进鸡蛋里。

turn one's back on / upon...
背弃……， 抛弃……

例句：
One should never turn his back on his home country.
一个人永远也不能背弃自己的祖国。

Nothings the matter.
一切正常，没问题
　　
对话：
A: It's too quiet today. Is there anything wrong?
B: Nothings the matter. It's just because everybody is too tired to talk.

A：今天真是太安静了，出什么事了吗？
B：没什么，只是因为大家都太累了，不想说话。