Sally is one of my friends who are coming to the party.
Sally is the only one of my friends who are coming to the party.
The first sentence is correct. The second sentence is not: it contains an error in subject/verb agreement.
In the first sentence, the antecedent of the relative pronoun “who” is “friends.” “Who” is therefore plural and it agrees in number with the plural verb “are.” The subject is one of many, and many are coming.
In the second sentence, the addition of the words “the only” makes the antecedent “one.” “One” is singular, so the verb should be singular. The subject is the only one who is coming.
When you encounter a noun phrase with the structure “one of [plural noun] who,” look for the word “only” before the noun phrase. If it is not present, the verb that follows should be plural because it should agree in number with the object of the preposition. If “only” is present, the verb that follows should be singular because it should agree in number with “one.” If you see “only” in front of such a noun phrase, consider crossing out the prepositional phrase “of [plural noun]” to make it easier to determine the agreement between the subject and the verb.
This is the corrected sentence:
Sally is the only one
of my friendswho is coming to the party.
Read Grammar Examples of this type of structure.
