1.- We can't take out the relative pronoun to understand the sentence.
Sally is the student whose marks are always the best.
2.- We can take out the relative pronoun to understand the sentence.
I understand the problem that you have.
3.-We can use that instead of which.
It's a journey which takes about 10 hours.
4.- The relative clause is separated from the whole sentence by commas.
Kerala, which is an independent state, is well developed.
5.- We can't use that instead of who.
Mary, who is my best friend, lives in Melbourne.
Northern Ireland is the country where my family live.