# Prime numbers are what “are” left when you have taken all the patterns out

I am studying the English grammar for an exam, and the book tells me what I thought was right is actually wrong. So I was wondering if I could get some help here.

In the sentence 'Prime numbers are what are left when you have taken all the patterns out.', why should are in bold be is? So that it is 'Prime numbers are what is left when...'

I thought it was correct already since the subject is in plural.

• Unclear -- which are it -- "are" or "is"? – Hot Licks Dec 24 '19 at 12:54
• I was thinking 'are' should come after 'what', but 'is' was correct, so I was wondering why 'is' should come. – Linus Dec 26 '19 at 3:05

It makes a lot more sense in context - particularly considering the character of Christopher from Mark Haddon's The curious incident of the dog in the night-time. Without giving too much away, the narrator is looking at patterns as something that could be problematic. This is why Prime numbers appeal to him.

In this case, the relevant noun looks like it should be "numbers" (which is plural), but it isn't. The relevant noun is "what" (which is singular).

Someone other than Christopher might have phrased it differently :

Prime numbers are whatever is left when you have taken out all the patterns.

Christopher is using the phrase [Prime numbers] to indicate a concept - or in mathematical terms a set. "Prime numbers" would be plural, but "the set of Prime numbers" is singular.

• Oh it was a line from a book?! I did not know that, but if that's what it's like in the context, thanks a lot. Ugh grammar can be as hard as it is. – Linus Dec 26 '19 at 3:08

//'Prime numbers are what are left when you have taken all the patterns out.'//

Because the 'left ones' are more than one number. If only one number was left, it would say, 'Prime number is what is left when you have taken all the patterns out.'

Let's change the order: "What are left after you have taken all the patterns out, are prime numbers.", Here, "What is left after you have taken all the patterns out, are prime numbers", can be used when the user is not sure of the count of what is left.

• Upvoted for the second rearrangement which clarifies nicely, but I think it's more about singular concepts and sets than the difference between "less" and "fewer". – ItWasLikeThatWhenIGotHere Dec 24 '19 at 11:15
• Hmm... in the second paragraph, to me it makes more sense to say "What is left after...". So I guess both are correct in that case? – Linus Dec 26 '19 at 3:06