She is only one of the girls that speak English.

We use 'only' to emphasise that something, e.g. a quantity or measurement is small or limited. It is necessary if you wish to suggest that she is not or may not be typical, in some way, of the girls that speak English, and that it would be unwise to draw some inference from her about the other girls that speak English.

John: Men that drive red cars are stupid.
Mary: How can you justify saying that?
John: Bill drives a red car and he is stupid.
Mary: But he is only one of the men that drive a red car.

George: French towns that are by the coast are dirty.
Jane: Why do you say that?
George: Dieppe is dirty.
Jane: But Dieppe is only one French coastal town.

Frank: Your sons are cowards.
John: What do you mean?
Frank: Your son James ran away from a barking dog yesterday.
John: James is only one of my sons.

You might say that she is "just" one of the girls who speak English. "That" is not incorrect, but i think a native speaker would be more likely to use "who."

Thanks , contrex and Setanta.

Con, s/b "speaks" ?

Yes, speaks, not speak.

No. Speak. Verb agrees with closest noun.

Speaks. Verb agrees with subject. I have a feeling that there's something new going on with the language. Not crazy about it.

Thanks Rob, it's not everyday....

The question is, how many of the girls speak English. If at least several of the girls speak English, then the sentence is correct as stands, and
only" is optional. If, however, the other girls do NOT speak English, and she is the only English speaker among them, then it would be "She is the only one of the girls that speaks English". How many English speakers do you have that you are referring to?

She is one of the girls who speak English.

She is among the girls who speak English

She is only one of the girls who speak English.

Yes. the sentence, as it stands, says that there is a group, "the girls that speak English", and that she is only one of them.

or, add the missing the, She is the only one of the girls who speaks English. Then speaks would fit, but I think not as the sentence stands without the the.

Thanks Oss. Wow but this stuff gets involved

I think some may be confusing the way English speakers use 'each' with 'one.' I think you could alternatively say, "of the girls, she is [only] one that speaks English," which emphasizes this particular girl over the rest. There's less ambiguity when rearranged this way.

Oops. Misread and gave the wrong info. My bad.

To anyone who doesn't know, Roberta knows her donuts about wordage, big time, and rarely errs, at least re american english.

She is high up/top on my most loved list. Be well and prosper, Robbie.