By no means true that all native speakers of English know their own language very well.
Sarah hopes to become a friend of _____ shares her interests.
_____ comes to see me, tell him I’m out.
Tell _______ you like — it makes no difference to me.
“It’s said that he is a wise leader.” “Oh, no, he is _____ but a wise leader.”
There are four bedrooms, ______ with its own bathroom.