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.

[EX Q]: Question: Is singular correct, or is plural of "credential" correct for a single username & password pair? 
Answer: This isn't a question you can answer by thinking about the grammar - it's just a case of conventional usage. I've previously worked in the tech industry, and we would always say credentials, never credential. That would still be true even if the credentials were a single item like a certificate. [Edit to add: but Samuel has provided a counter-example to this in a comment on the OP.] In wider English, credential is valid - it's used for example in this Wikipedia article - but it's still more common to use the plural (see Oxford Dictionaries for confirmation). For example, I could say that I have presented my credentials to someone if I have shown them my passport, even though I've only shown them one physical item.
[EX A]: yes

[EX Q]: Question: This is a question for English wordsmiths. Is there a word that "feels" like the word "ethereal" that means something ethereal that quickly disappears. Or is "ethereal" the correct word choice for this? Ethereal was the first word that popped into my head for this. From the various definitions that I've looked at for ethereal, it says delicate, beautiful, heavenly, etc. but not necessarily quickly vanishing, except for the secondary definitions related to ether that suggests it evaporates. I'm just wondering if there is a better word than this. I'm using the term "ethereal skill" to describe a certain type of pilot skill (instrument flying) that requires tremendous practice to achieve proficiency, and once proficiency is achieved it is a beautiful thing, but if you don't practice it regularly, that skill vanishes after a few weeks. USAGE: Instrument proficiency is an ethereal skill that once obtained must be maintained through constant and regular practice. 
Answer: evanescent is a good match - definitions focus on loss and vaporous qualities. It seems to be applied most often to the loss of things with positive connotation. MW defines it with : "tending to vanish like vapor", Cambridge with "lasting for only a short time, then disappearing quickly and being forgotten" Century Dictionary Online has a number of useful definitions and quotes. I like "Vanishing or apt to vanish or be dissipated, like vapor; passing away; fleeting: as, the pleasures and joys of life are evanescent. Also from Century, this quote: "He [Wordsworth] seems to have caught and fixed forever in immutable grace the most evanescent and intangible of our intuitions, the very ripple-marks on the remotest shores of being." - Lowell, Among my Books.
[EX A]: no

[EX Q]: Question: If I were to try to achieve something you could say I "had a go". If I tried it multiple times, how would I write that down? I had many goes or I had many go's or I had many gos 
Answer: Not what the OP intended... The noun go (a Japanese board game) has the English plural gos. Perhaps one could even say "I had a go" with this meaning.
[EX A]:
no