" One of you ____ is the thief." Which verb - "is" or "are" - should be used in the blank space in the given sentence? Some of my friends are in favour of "is" for the simple reason "one" is the headword of the subject -noun phrase: "one of you". But another senior colleague of mine authoritatively states that "are" is the correct option:' As the sentence is directed at a group of persons, "you" should be considered the headword of the subjec.' Kindly clarify.
One (of you) is the thief.
One is the thief.
One person within the group is the thief.
Only one of you is the thief.
'One' functions as the subject, and 'of you', as its modification. Modification doesn't agree in number with the verb. Your colleague may be thinking of correlative conjunctions, (n)either . . . (n)or; e.g. Neither you nor I am the thief. With those constructions the verb agrees in number with the closest subject.