He didn't know how to solve this problem.(保持句意基本不变）He (空三格) this problem.
问题描述：
He didn't know how to solve this problem.(保持句意基本不变）
He (空三格) this problem.
答
he was confused with this problem
他正为这个问题所困扰
答
can't work out
答
can't deal with
答
He didn't know how to solve this problem.(