they know nothing at all about the difficulty we had in doin

他们根本就不知道那时我们在做件事时所面临的困难.
本句中包含一个以difficulty为先行词的定语从句,另外有一个短语也是考点,即 have difficulty (in) doing sth.本短语中in可省略
这个复合句可拆分成两个简单句
They knew nothing at all about the difficulty.
We had difficulty in doing the work then.