do you think it necessary to do my homeworkDo you think it necessary to do my homework为什么不是Do you think it is necessary to do my homework

do you think it necessary to do my homework
Do you think it necessary to do my homework
为什么不是Do you think it is necessary to do my homework

这两个句子都是对的,意思也相同.不同的是第一句中it在句中做形式宾语,后面的不定式是真正的宾语,第二句it及以后的部分做think 的宾语,it在宾语从句中做主语.