The key is to practice,which I think of great value to our study.

有一点点小问题,加一个is
The key is to practice,which I think 【is】 of great value to our study.
关键是练习,我认为这对我们的学习来说是有很大价值的.可是 I think it of great value to our study。是正确的表达，而且介词作宾语补足语是符合规范的。这也是《高中英语基础知识手册》上的例句啊？？？？？？？？？求解释这是think的另一种用法 think sth. + 宾补I think it of great value. 我认为它有巨大价值。如果是I think it is of great value. 则是think后面加宾语从句的用法也就是说原句变成定语从句后表达无误喽?没有定语从句原句是对的