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.

he was confused with this problem
他正为这个问题所困扰

can't work out

can't deal with

He didn't know how to solve this problem.(