1: Do you know them? "you" = subject "it has semantic content that is in the shared knowledge of the speaker and the hearer"
Like how you did in your example = you do know them, do you?
2: I consider him a genius. "him" = dummy subject, who is him?
*him I consider a genius,do him?
I hope I am right this time with dummy subject?
What I understand is:
It is a hot day.
It is difficult for everyone to master English.
It is kind of you to help us.
It is worth reading this book.
It takes me five minutes to go to shcool.
There are three persons in my family.
There developed goodwill between the two countries.
The bold words are dummy subjects.
After reading all these great examples from all of you, I hope I am right this time!!!
1: It is difficult to please everybody.
"it" = dummy subject. = what's difficult?
"It" is an expletive: It is not meaningful
just like the example in - It is raining = What's raining?