The medicine always made him feel dopey and unable to concentrate.

那药总让他感到昏昏欲睡且无法集中注意力。

《柯林斯英汉双解大词典》

I was unable to concentrate on studying and for a time, I was even going to just pack my bags and leave for home instead of sitting those exams.

我无法专心学习，有段时间，我甚至想收拾行李回家去，而不是坐在这里考试。

youdao

Are you tired or unable to concentrate during the day?

你是否在白天时感到疲惫或是无法集中注意力？

youdao

The victim said the abuse had left him regularly depressed and unable to concentrate.

BBC: Ex-Isle of Wight choirboy compensated for priest abuse

His 15-year-old brother is taking high school exams this month, but says he is unable to concentrate.

WSJ: Victim's Family in India Still Reels

He became forgetful about discussions and appointments, and he was unable to concentrate or focus, the suit said.

CNN: January 24, 2013 -- Updated 1044 GMT (1844 HKT)