This is the easiest one to sort out. "Who" is used for people (and occasionally for animals being treated as humans).
- Who is going with you?
- 'Last of all came the cat, who looked round, as usual, for the warmest place…' (from George Orwell's Animal Farm.)
This is also the case with "who" when used as a relative pronoun:
- The person who finishes first is the winner.
What or Which: Which to Choose?
This is a trickier choice. Basically, the use of "which" implies a choice of a few (up to about four) options.
Which do you like better: red or green? (This is a clear choice of two options.)
Which bus are you getting? (where a few buses are available).
There are four umbrellas here. Which is yours?
Of course, we still use "who" with a choice between people:
Who do you like better: Mr Jones or Miss Smith? (NOT "Which do you…")
although "which" is used if a noun is included with it. For example:
Which doctor did you see?
Which idiot said that?
"What" introduces a more general question, where the options are numerous.
What colour do you like best? (This gives a choice from any colour one can think of.)
What day is it today? (Seven options is considered more than "a few" to choose from.)
"What is the longest river in the world?" asks for a choice from all the rivers in the world.
Compare this with:
"Which is the longest river – the Amazon, the Mississippi or the Nile?"
What + noun
Again, these imply general questions:
What size is this skirt? suggests there are many options.
(Compare this with:
Which size is this skirt? This implies there are, say, only small, medium and large skirts to choose from.)
Some further examples:
What time is it?
What nationality are you?
What make is your mobile phone?
Fill in the blanks with what, which or who in this short exercise.
Which pronoun causes you problems? What's your final score? Who can get 10 out of 10?