OK, here is an example I had in a recent “Business Grammar” class:
“I bought one of those watches that never needs winding.”
The instructor said that “needs” is proper in number, becase “that” is referring to “one” which is singular.
I disagreed, thinking that the statement as written was ambiguous. In my opinion, if the conversation was about watches, then the “of those watches” has little importance. The sentence with it’s salient parts would then be “I bought one (watch) that never needs rewinding.” OK, no problem there.
However, if the conversation was about something else, and I just happened to think about my recent purchase, then the salient part would be “those watches that never need rewinding,” requiring a plural verb, because that is the main point.
Diagrammatically, in the first example, “of those watches” would be and adjective prepositional phrase modifying “one.” “That never needs rewinding” would be a adjective clause modifying “one.”
In the second example, “of those watches that never need rewinding” is an extended prepositional phrase modifying “one,” and “that never need rewinding” is an adjective clause modifying “watches.”
Once again in recap, the instructor said that the first way was correct. I thought that it could be either way, depending upon the context.
(I also think somehow this question ties in to the ga/wa confusion in Japanese.)