Q: This grammar question was posed by a friend on Facebook: Which is correct? (1) “She is one of the few freshmen who understand” or (2) “She is one of the few freshmen who understands.” At first I thought #2 was the answer. Now I’m not sure.
A: The first example is correct. The verb here is plural (“understand”) because the subject (“who”) is plural. It refers to the plural “freshmen.”
You might mentally transpose the sentence this way: “Of the few freshmen who understand, she is one.”
Sentences with “one of the” can create verb agreement puzzles, as we wrote back in 2007.
The problem here is compounded by the presence of “who.” We wrote in 2014 that “who” can be either singular or plural. When it’s preceded by a noun (as in “freshmen who”), “who” takes its number—singular or plural—from the preceding noun.
Another common problem crops up when we use “one of the” and “if not the” in the same sentence.
Say you go to a fantastic pizzeria and conclude, “That was one of the best, if not the best, pizza I’ve ever had.” Then you wonder if the noun should have been plural, “pizzas.”
The trick here is to put “if not the” toward the end of the sentence, after the noun: “That was one of the best pizzas I’ve ever had, if not the best.”
Here’s how Pat explains it in her grammar and usage book Woe Is I (3rd ed.):
“ONE OF THE . . . IF NOT THE. Here’s another corner you can avoid backing yourself into: Jordan was one of the best, if not the best, player on the team. Oops! Can you hear what’s wrong? The sentence should read correctly even if the second half of the comparison (if not the best) is removed, but without it you’ve got: Jordan was one of the best player on the team. One of the best player? Better to put the second half of the comparison at the end of the sentence: Jordan was one of the best players on the team, if not the best.”
Finally (since we brought it up), “if not” in this case means “perhaps” or “maybe even.” That’s generally the case when used with superlatives like “best,” “fastest,” “oldest,” and so on.
But as we wrote in 2013, “if not” can also mean “but not,” as in “His language is colorful, if not grammatically correct.”
from Grammarphobia http://www.grammarphobia.com/blog/2016/09/one-of-the.html