The relative pronoun which is used to refer to objects and animals. It cannot be used to refer to people. Which has the same form for the nominative (subject) and the accusative (object) case.
Which has no possessive form. But if we really need to express that idea, we can use a structure with of which. In a less formal style, we can express the same idea using whose + noun.
Note that whose can refer back to people or things. It can replace the pronouns its, their, hers and his.
He has a beautiful sister. I have forgotten her name.
He has a beautiful sister whose name I have forgotten.
The structure with which is not possible here because it cannot refer back to people.
It was a meeting. I did not comprehend its importance.
It was a meeting whose importance I did not comprehend.
It was a meeting the importance of which I did not understand.
The umbrella the handle of which is broken was bought only last week.
The umbrella whose handle was broken was bought only last week.
What has the same form in the nominative and in the accusative cases and is used only in the singular. What means ‘that which’ or ‘the things which’.
What cannot be cured must be endured. (= That which cannot be cured must be endured.)
I have got what I wanted. (= I have got the things which I wanted.)
Note that clauses beginning with what act as the subject or object of the verb in the main clause. For example, in the sentence given above, the clause ‘what I wanted’ is the object of the verb got.