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

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

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

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