They didn't know what they should do to deal with the problem.改为简单句
They didn‘t know what __________to deal with the problem.

答案解析如下：关系代词what +主语+谓语+其他
可以改为 what +动词不定式变成简单句
请看下面

They didn't know what 【to】【do】to deal with the problem

他们不知道做什么来处理/应付这个问题 to deal with the problem 是动词不定式做目的状语