relative clauses

The sentence "I don't remember the street where we parked the car" is non-essential.

The sentence "Dennis, the boy that doesn't want to go, is sad" is correct

The word “which” in “My neighbour has a dog which barks at nights” can be replaced by “that”.

The sentence “What's the name of the person who is singing?” is essential.

The sentence “This is the key /ki/ that opens the door.” is correct.

The sentence “This is the restaurant where we usually have lunch.” is correct.