they all felt sleepy because they found the lecture hard----to have been understood为什么不对?不是found吗?过去时啊不定式也有时态的亚不过我问的是为什么have不对，答案只是to understand如果to have understood呢
问题描述：
they all felt sleepy because they found the lecture hard----
to have been understood为什么不对?不是found吗?过去时啊
不定式也有时态的亚
不过我问的是为什么have不对，答案只是to understand
如果to have understood呢
答
find sth. to do
do不因find的时态改变而改变,因to+原形do。
they all felt sleepy because they found the lecture hard to understand
一般understand不用于完成时态。
答
to是动词不定式后面跟动词原型
关于不定式时态
过去时 to do
完成时 to have done
此句逻辑主语是They
所以understand的语态要与逻辑主语一致,不用被动
补充：因为此处不定式做宾语补足语,与时态无关