这是一个用that引导的定语从句.
that在句中是关系代词,用来指代：Another thing.
定语从句部分“ that he found very difficult ”,用来说明Another thing.
整个句子翻译成中文为：另一件他所发现的难事是英语语法.
主体是：Another thing was English grammar(另一件事是英语语法）,
说明部分是：he found very difficult (他发现困难的)