Final answer:
The second 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 second 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, it serves as the main verb that expresses the action or state of being in a sentence. For example, in the sentence "She runs every morning," the word "runs" functions as the main predicate, indicating that it is a verb.