WHO
The subject of the sentence:
The man who bought my car.
Nobody who has really thought about it can possibly agree...
They caught the accountant who stole the money.
WHOM
Technically the object of the sentence.
The man to whom I sold my car.
Whom did you give it to?
but
In modern English, in conversation it would be more normal in the above examples to say
The man I sold my car to. (Note that "to" has to be at the end)
Who did you give it to?
In written English, or formal situations it is better to use the "correct" form:
Our benefactor, to whom we are infinitely grateful, has once more come to our rescue.
The directors, whom I know personally, are all behind me in this.
WHICH
Use it for things, object or subject.
The injection which I gave him was for Polio.
The injection, which was painless, was for Polio.
The injection which I recieved was for Polio
WHOSE:
Posessive:
I spoke to a man. HIS office is near mine.
I spoke to a man WHOSE office is near mine.
The woman cancelled HER appointment. It was at 3:30.
The woman, WHOSE appointment was at 3:30, cancelled it.
Note that "whose" can be used for objects, not just people:
TMA is a company. ITS products are sold here.
TMA is a company WHOSE products are sold here.
Sometimesit is possible to
use OF WHICH instead of
WHOSE:
The organisation, the problems of which we have all heard, has little influence these days.
or
The organisation, whose problems we have all heard, has little influence these days.
But not always:
It is a problem of which we are all aware.