I would like to know when we should say what and when we should say which. Why do you use what and which in this example instead of only what?
which / what
What and which are interchangeable in your example Telma. Both what or which would fit in both places with little or no difference in meaning. Often what and which are used for the sake of variety. The same is true in these examples of direct and indirect questions:
* Which / What would you say are the most polluted cities in the world and which / what are the cleanest?
* Do you know which / what sort of plants grow best in a shady garden?
* I've no idea which / what road to take to Jimmy's place.
~ What / which route did you take?
However, when we are choosing between just two or three options, we usually prefer which. If there is no limit to the number of choices, what is used. Compare the following:
* What would you like in your sandwiches? I've got cheese or tuna. Which would you prefer?
* He comes from Glasgow, but do you know which football team he supports?
~ I think it's Celtic, but it may be Rangers. I'm not sure. What / which football team do you support?
* What is your postal / email address?
* What's your opinion on this?
* Which TV channel is the tennis on?
what / which - before nouns
Before nouns what and which can be used interchangeably to ask questions about people or things:
* What / Which colour trousers would you like? Brown, green, blue, orange or maroon?
* Which / What writers have made the biggest impression on you?
which - before 'one' and 'of'
However, if we wish to use the which of or which one constructions, our choice is limited to which:
* Which of these cars are you interested in driving?
* Which ones should I choose?
* Which of these teams do you think will win the championship?
* Which one of us is going to make the presentation?
who / what / which - without nouns
Note that when these words are used as pronouns with no nouns immediately following, we usually use who when referring to people:
* Who do you think will win the championship?
* Who will take over as captain if both Keane and Beckham are injured?
* Who are you going out with now? Is it Leslie or Keith?
* Which footballer would you like to go out with?
However, if we are trying to identify certain people out of a group of people (e.g. in a photograph or in a crowded room), we use which, which is similar in use to which one(s):
* Which (one) is your boyfriend?
~ The one (who is) sitting next to Fiona.
* I need to know which (of these) children have not been vaccinated.
If we are asking about someone's job or function, we can use what or which:
* What / Which would you rather be - a general doctor or a specialist of some kind?