WHO - WHOSE - WHOM - THAT - WHICH - WHERE

There is often confusion about the use of who, whose, whom, that, which or where. 

• We use who when referring to people or when we want to know the person.
  • The person who answered the phone was very helpful.
  • Who ate all the chocolates?
• We use which to refer to a thing or an idea, and to ask about choices.
  • My car, which is 20 years old, isn't worth much.
  • Which size would you like, small, medium or large?
• We use that for both a person and a thing/idea.
  • I'm talking about the person that I saw yesterday. 
  • This is the style that I want to use.
• Whose refers to ownership.
  • Whose dictionary is this?
  • There's the girl whose car was stolen.
• Whom: 
  When who is the object of a verb, whom, with a preposition, can be used instead,
  but it is formal and rather old-fashioned. In modern speech, we use who, or we leave out
  the pronoun.
  • You are referring to a person who no longer works here.
  • The person to whom you are referring no longer works here.
  • The person (who) you are you referring to no longer works here.
  
  Whom is always used when it is preceded by quantifiers such as all of, both of, few of,
  many of, several of, etc. For example:
  • He addressed the spectators, most of whom remained seated.
• Where (relative adverb) refers to places and locations.
  • Where is the station please?
  • That's where I spent my childhood. 

Examples of use :

I know a woman. She speaks 6 languages.	I know a woman who speaks 6 languages.
I know a woman.  Her husband speaks 6 languages.	I know a woman whose husband speaks 6 languages.
I spoke to a person yesterday.	The person to whom I spoke yesterday.(formal) The person (who) I spoke to yesterday. (informal)
I live in a house.  It is 200 years old.	I live in a house which/that is 200 years old.
That's the hotel. We stayed there last year.	That's the hotel where we stayed last year. That's the hotel that we stayed in  last year.

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When can we leave out relative pronouns  (who, whom,  which, that)?

In conversational English relative pronouns can be omitted when they are the object of a relative clause. In a formal context it is usually wiser to leave the relative pronoun.

Subject	Verb	Object
Tom	drives	a red truck
→ The person who drives a red truck is called Tom. In this sentence 'who' refers to the subject so it cannot be omitted. → The truck (that) Tom drives is red. In this sentence 'that' refers to the object (the truck) so it can be omitted.

Compare :

• The woman who wanted to see me is a doctor. ('Woman' is the subject of the sentence)
• The woman (that) I wanted to see is a doctor. (Here 'woman' is the object, 'I" is the subject.)