Relative Pronouns

A
"who" relates to a grammatical SUBJECT which is a PERSON, not a THING:
   
I know a man
who
has a pet gorilla.
   
What's the name of that girl who lives in Durach?
   
There are many old people who live alone.

B
"that" or "which" relates to a NON-PERSON subject or object:
   
Where are the eggs
which

that
 were in the fridge?
   
That's a problem  is difficult to solve.
   
Learning to play the piano well is a skill  takes many years to learn.

C
a QUESTION WORD can be used in a similar way:
   
I don't know where he lives.
   
I don't know how he does it.
   
Do you know what he wants?
   
Can you tell me which way to go?

D
"whose" relates to the OWNERSHIP. Remember the question: "Whose is this book?"
   
Do you know
 whose 
this is?
   
There's the boy father is a champion boxer.

E
Using "SEE" and "HEAR" in the PRESENT CONTINUOUS - you can LEAVE OUT THE WHO+AUXILIARY:
   
Can you see that lady who is crossing the road?
   
I can hear some birds which are singing.