Well, I´ve been tring to learn about it. Is this right?
You cannot say: I´ve read the letter that was safely kept, because the boss had gone at that moment.
You must say: I´ve read the letter, which was safely kept, because the boss had gone at that moment.
If you add some extra information in a non-defining relative clause, you can leave out “That” because it is not taking the subject place:
Ghosts, whom I´ve never been afraid of, have always had an eery appearance.
It could be different to say: Ghosts that you have spoken of are only real in your dreams.
I am not sure at all of this. Please, give me your opinions.
Best regards, the greatest friends.