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。