Sentence: A group of words with a subject and verb that express a comlete thought. (Should I use the word expres or expresses in this sentence and why?

Question

anonymous · Answer

So this one is somewhat controversial.

If you construe the simple subject of "that expresses" to be "group" -- it is the group that expresses the complete thought -- then you would use the singular: expresses.

If you construe the subject of "that expresses" instead to be "a subject and verb" -- such that it is what is within the group of words that expresses the complete thought -- then you would use the plural: express.

Generally, whenever you have a construction with a prepositional phrase (here: "of words"), then the subject of the verb will be the noun that governs that prepositional phrase (here: "group"). But the complication in this sentence is the second prepositional phrase, "with a subject and verb."  Ought this phrase to be construed as modifying "words" or "group"? You could argue either way.

It's difficult to explain this more clearly without a sentence diagram, but I mean something like this --

     A group [of words ---> with a subject and verb] that expresses . . .

     A group [of words] with a subject and verb that express . . . 

Does that help clear things up?