I’m afraid that you didn’t get it right, lv. From the grammatical standpoint, the correct antecedent of the subordinate clause “that expresses a complete thought” is the singular noun phrase “group of words”; this is the reason why “expresses,” the operative verb in that subordinate clause, is in the singular form. Also, from a notional standpoint, what “expresses a complete thought” isn’t just “a word” or just “any group of words,” but “a sentence” specifically, so the subject being modified by the subordinate clause “that expresses a complete thought” could only be the group of words that’s known as “a sentence.” Both grammatically and notionally, therefore, the operative verb in that sentence should be in the singular form “expresses.”
Determining the correct antecedent noun in sentence constructions of this kind is admittedly tricky. It’s very tempting to assume that the noun nearest to the operative verb is the antecedent noun, but this isn’t the only criterion in establishing subject-verb agreement; we need to verify if that assumption is borne out by the semantics and logic of the sentence. In this particular case, it is semantically and logically clear that the antecedent noun—the subject that’s being modified by the subordinate clause—could only be the singular noun “sentence.”
