Another very difficult thing that I found was English grammar这个句子没有犯语法错误,只是我们不会这么说.
在Another thing that I found very difficult was English grammar这个句子中,其实考查的不仅仅是定语从句,还有find sth.adj这个结构.find在find sth.adj这个结构中意为“认为,觉得”.原句要表达的意思是“我认为另一个比较难的东西是英语的语法”.而在“Another very difficult thing that I found was English grammar”中,我们会自然而然地把found（find）当作“找到”讲.