对的,a waste of time to attend classes 作为it的宾语补足语
当然,加上is也是对的,但就变成了另外一种表达方法,变成了一个从句
think后面是个从句,a waste of time是个动词短语浪费的意思,不是冠词“a”。is是be动词,一个单句不能用两个动词。
think 后面宾语补足语用法是:
1. think + object + (to be) + n./adj.;
2. think + it + adj. + to do.
所以最好应该是用宾语从句,也就是加is.
It's waste of timeto attend classes, they think, because some textbooks are out of date.