什么时候用it什么时候用it is,如I think it difficult to do my homework.

问题描述:

什么时候用it什么时候用it is,如I think it difficult to do my homework.

应该是I think it is difficult to do my homework.
is是谓语

这句都可以
如果看成从句就是I think that it is difficult to do my homework
省略that就变成I think it is difficult to do my homework