We always use a relative clause beginning with whose + noun, particularly in written English, when talking about something that belongs to or is associated with a person. Compared:
Example (1): Stevenson is an architect. His designs have won international acclaim.
Example (1): Stevenson is an architect (whose) designs have won international acclaim.
Example (2): Dr. Rowan has had to write everything himself. His secretary quit two weeks ago.
Example (2): Dr. Rowan, (whose) secretary resigned two weeks ago, has had to type everything.
We can use ‘whose’ in both defining and non-defining relative clauses. We sometimes use ‘whose’ when we talk about things, in particular when we talk about cities or countries and organisations:
Example: The film was made in Botswana, (whose) wildlife parks are larger than those in Kenya.
Example: We need to learn from companies (whose) trade is healthier than ours.
Example: The newspaper is owned by the Mearson Group, (whose) chairman is Sir James Box.
We can also use ‘whose’ when talking about particular items, although it is often more natural in spoken English to avoid sentences like this:
Example: I received a letter, (whose) bad spelling made me think that it was written by a child (it would be more natural that I received a letter, and its bad spelling…)
We often use the words where, when, and why as relative pronouns. But in formal English in particular, a preposition + phrase can often be used instead:
Example: This was the place (where) we met for the first time (or… the place in/in which… )
Example: I did not look forward to the time (when) I would have to present evidence in court (or… the time I would have…)
Example: Do you know the date (when) we have to submit the first essay? (or… the date on/when we have to submit the first essay?)
Example: The government should end the system (whereby) farmers make more money by leaving land fallow than by growing wheat (or… the system where farmers…)
We can also use ‘why’ as a relative pronoun after the word season. In formal English we can use ‘that’ instead of ‘why’:
Example: I did not get a raise, but this was not the reason I left. (or… the reason (why) I felt it.)
Sometimes we use relative clauses that start with who or what. In this case, ‘who’ means ‘the people who’ and ‘what’ means something like ‘the thing(s) that’:
Example: Can you give me a list of who has been invited?
Example: I didn’t know ‘what’ to do next.
Note that we can’t use ‘what’ in this way after a noun:
Example: I managed to get all the books (that) you asked for (not… the books that you asked for).
Relative clauses beginning with ‘whatever, whoever or whatever’ are used to talk about things or people that are indefinite or unknown:
Example: I’m sure I’ll enjoy eating (whatever) you cook.
Example: Whoever wins will play against Barcelona in the final.