GRAMMAR
We can use RELATIVE CLAUSES to identify, define, or classify a person or thing. The relative clauses fallows a noun.
In relative clauses we should use:
Who: When we talk about people.
For example: He has a lot of friends who is very generous.
Which: When we talk about things.
For example: My brother bought a smartphone which is very luxurious.
That: When we talk about things and people.
For example: I work with people that are responsible.Whose: When we talk in possessive forms.
For example: My students whose homework is never done. will fail the exam.
The RELATIVES CLAUSES are important because we can specify the circumstances of persons, animals or things