We need to ask two questions about these examples.
what is the function of the adjective clause in each example? Second, how do English speakers
use the construction(s) we´re looking at? The second question is as important as the first one.
In S1 "who are American" is a restrictive adjective clause that refers to a subset
of students, a part of a larger set of students. It can be paraphrased as
Of the students who are American, one was late.
For this reason, S1 is correct as written.
S2 and S3 (the adverb only makes no difference to the grammar) are not correct as written.
In logical terms, they cannot be paraphrased as
Of the students who is American, [only] one was late.
Can we show that only one student is American and that
this student was late by using an adjective clause? No.
Even if the adjective clause is punctuated as nonrestrictive, with commas:
[Only] one of the students, who is American, was late.
the sentence may imply that the student is the only American in the group, but it does
not state that idea. The only way to show that only one student is American and that
this student was late is this:
The only one of the students who is American was late.
This version, of course, gives a different meaning and function to the pronoun one.
Now for the important question: Is this a natural use of such a construction? In the past tense
and past perfect, verb forms in which the number of the subject is not marked on the verb, this
construction occurs in all positions: subject, direct object, object of preposition, and subject
complement. However, in illustrating the choice of number on (present tense) verbs, both Quirk
et al. (A Comprehensive Grammar of the English Language, Longman, 1985) and Biber et
al. (Longman Grammar of Spoken and Written English, Longman, 1999) use as examples
only sentences in which this construction is the subject complement (the noun phrase
that follows the verb be or other similar verb).
Now we have a real choice or
do we? Quirk et al. state (Section 10.43, note [d]): "The choice of singular or plural [verb]
can depend on whether attention is directed to the generality or to the uniqueness."
(Grammar Exchange italics)
Let´s take some examples from the COBUILD corpus* of subject complement use:
Focus on the generality:
He is not one of those officers who follow orders blindly and unthinkingly. [The
focus is on the set of officers.]
Focus on the uniqueness:
If you are one of the thousands who has already renewed your subscription
[The focus is on the addressee, the
individual member of the larger set.]
Biber et al. state that the use of a singular verb "should probably be ascribed to the pull
of the numeral one toward the singular
combined with the fact that the main clause
makes reference to a single person" (Section 3.9.3).
So, returning to the examples at the beginning, it would be
possible to have a sentence such as:
She is one of the students who is American (in every
thus emphasizing her uniqueness.
*The Cobuild Collocation Sampler can be found at the