think后面加宾语从句比较多见.I think you are stupid.I think I was wrong.
可以加状语,如I think so.I think badly,I think hard.
可以跟that的宾语从句,一般表达比较复杂意义的时候用.I think that the TV was plugged incorrectly and that is why no signal was recieved.
I think it is应该不是什么特别的,只是用it is开头的从句做了宾语就是了.