«Excuse me! What’s the difference between what and which?» Aren’t you tired of asking (if you are a student) or being asked (if you’re a teacher) this question? I am, for sure. However, it is perfectly understandable that many learners have problems with these two, as they are, in truth, quite similar. For this reason, this post is organised into similarities and differences as regards use and meaning.
Similarities
1. Both of them are interrogative pronouns that can go at the beginning of a direct or embedded question:
What‘s your name?
I’m not sure what you mean.Which are the best varieties?
I don’t know which one to pick.
2. Both of them can work as relative pronouns, joining two separate clauses:
What they needed was someone to clean their house.
«The Lord of the Rings» is what I am reading at the moment.It was a crisis for which he was totally unprepared.
She went to a conference in Vienna which ended on Friday. (Source: Oxford Dictionaries)
3. When acting as interrogative pronouns, they are sometimes interchangeable:
Which / What would you say is the most beautiful city in the world?
I’ve no idea which / what road to take to Jimmy’s place. (Source: BBC)
Differences
1. When acting as interrogative pronouns, if the question has a limited number of answers, that is, a limited choice, we must use «which«. Note the difference between «what» and «which» in the following situation:
What would you like in your sandwiches? I’ve got cheese or tuna. Which would you prefer? (Source: BBC)
In the example above, we can see how in the second question, the choice of answers is limited to «cheese» and/or «tuna», therefore being compulsory the use of «which», not «what».
2. When working as relative pronouns, these two words are not interchangeable. The difference, one could say, is that «what» is less specific than «which». This is because «what» can usually be replaced for «the thing which», where «the thing» can actually be anything. Read the following examples for further clarification.
The «Lord of the Rings» is the book which I am reading at the moment.
The «Lord of the Rings» is what I am reading at the moment.
For this reason, I normally tell my students that you cannot use «what» if you specify what it is that you are talking about. For instance:
Ronda is the Spanish city which Hemingway fell in love with.
This is the hammer which was used to kill the victim.
In some of these cases, you could change the whole underlined phrase for «what», although you ought to be careful because this may sound weird in some sentences like the first one.
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