Final answer:
The third syntactic criterion to determine whether a word is a verb or not is the word's function as the main predicate in a sentence.
Step-by-step explanation:
The third syntactic criterion to determine whether a word is a verb or not is the word's function as the main predicate in a sentence. When a word functions as the main predicate in a sentence, it serves as the primary verb that expresses the action, occurrence, or state of being. For example, in the sentence 'She runs every morning,' the word 'runs' functions as the main predicate, indicating that running is the action being performed by the subject, 'she.'