Question: I'm normally pretty good with grammar, but this one just stumped me. I was trying to write something similar to "you're one of the only people who [is/are] nice to me", but I'm not sure if I should use "is" or "are".

Which sentence is correct, and why?

> 
> You're one of the only people who is nice to me.
> 
> 
> You're one of the only people who are nice to me.
> 
> 
>

Answer: 
To determine agreement, you need to find the head noun being modified by the relative clause *who is/are nice to me*. First let's find the **head noun** candidates . . .

> 
> You're one of the only [**people**] [who are nice to me].
> 
> 
> You're [**one**] of the only people [who is nice to me].
> 
> 
> 

Now let's test them . . .

> 
> You're one of the only **people who are nice to me**. --> Of the
>  only **people who are nice to me**, you're one.
> 
> 
> You're **one** of the only people **who is nice to me**. --> Of the only
>  people, you're **one who is nice to me**.\* (incorrect—which only people?)
> 
> 
> 

Sometimes you do have a choice! Here are some more **head noun** candidates . . .

> 
> Gruyère is one of the Swiss [**cheeses**] [that are used in fondue].
> 
> 
> Gruyère is [**one**] of the Swiss cheeses [that is used in fondue].
> 
> 
> 

Again, let's test them . . .

> 
> Gruyère is one of the Swiss **cheeses that are used in fondue**. -->
>  Of the Swiss **cheeses that are used in fondue**, Gruyère is one.
> 
> 
> Gruyère is **one** of the Swiss cheeses **that is used in fondue**.
>  --> Of the Swiss cheeses, Gruyère is **one that is used in fondue**.
> 
> 
> 

Deciding whether something (e.g. *one*) is a pre-modifying quantifier or a head noun can be tricky. I imagine any rule that might exist would be way more complicated than applying the tests. Meanwhile, you can always write your way out of the situation . . .

> 
> You're among the few people who are nice to me.
> 
> 
> 

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Further reading:

[Quantity Phrase Nouns—Agreement with Verb in Relative Clause](https://www.grammar-quizzes.com/agree4b.html#quantityphrase)
Correct Sentence is **You're one of the only people who are nice to me.** 

That is because in this sentence we check agreement of the *verb* **are** with the *plural noun* **people**. 

Here the [verb depends on agreement with the noun](https://webapps.towson.edu/ows/sub-verb.htm) i.e Plural Noun - Plural Verb or Singular Noun - Singular Verb