The man .................... is wearing a black hat is a spy!
The man who is wearing a black hat is a spy!
DEFINING
The men, ............ were questioned by the police, were allowed to go home
The men, who were questioned by the police, were allowed to go home
NON-DEFINING
THAT CAN´T BE USED AFTER A COMMA
That woman, .................. son is an actor, was cleaning the window
That woman, WHOSE son is an actor, was cleaning the window
NON-DEFINING
THE FACT THAT HER SON IS AN ACTOR IS ADDITIONAL INFORMATION
I won the competition, .......................... was amazing
I won the competition, which was amazing
NON-DEFINING
"WHICH WAS AMAZING" GIVES EXTRA INFORMATION. THERE'S ONLY ONE COMMA
The manager interviewed the candidates, all of .......... had excellent qualifications
The manager interviewed the candidates, all of whom had excellent qualifications
NON-DEFINING
IT GIVES ADDITIONAL INFORMATION.
AFTER A PREPOSITION YOU MUST USE WHOM (not who)