Definition: Given the Question and Answer pair, detect whether the answer is acceptable or not. Denote acceptable answer as "yes", otherwise denote as "no". An acceptable answer is consistent with the context in question, detailed, long, and uses general knowledge of English. Answers that are not definite and suggest a variety of options are not acceptable answers.
Input: Question: What is the plural of syntax? Would it be syntacies? What rule would govern this kind of construction? 
Answer: As FumbleFingers has commented, the plural is syntaxes. That's probably because the normal plural -es is added to the root word, or it may possibly be because the Greek/Late Latin word from which syntax is derived is syntaxis and Greek words ending in -is are pluralised with -es. If we were to create a spoof plural for syntax it would be syntaces (words ending -x get -ces; the preceding vowel can change as in index/indices). But no-one uses that, or would know what it meant.

yes