TASK DEFINITION: Generate the answer for a given question. The question requires common knowledge of English. Make sure that the answer is in detail, long, consistent with the context, and requires general knowledge of English. To be less repetitive, please vary your language for each question. Give a proper explanation to answer, and if necessary, provide the definition of various terms. And conclude your answer if it's long. If you have used other resources (e.g., Merriam Webster's.) to answer the question, include citations in your answer. You don't need to paraphrase the context of your resources, but make sure that your answer is consistent and complete.
PROBLEM: Is singular correct, or is plural of 'credential' correct for a single username & password pair?.

SOLUTION: 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.

PROBLEM: I've heard this joke several times, but I'm ashamed to say I really cannot understand it. It just doesn't seem to make any sense however I look at it. I have a suspicion that it is supposed to be rude, so I'd like to offer my apologies if it causes offence- that is not my intension. Here's the joke: Two nuns are in the bath. Nun1: Where's the soap? Nun2: Yes, it does rather. Edit: I understand the pun on 'where's'/'wears'. But what I don't understand is why that is funny. Several people have mentioned that there is a sexual inference, but I don't understand where it comes from.

SOLUTION: 'Why is this funny?' may be one of the hardest questions to answer. Humor is subjective, circumstantial, and, not infrequently, ineffable. Personally, I don't think I'd ever encountered a nun joke before this, as I didn't grow up in a community with many Catholics and nuns didn't really seem relevant to our lives. Thus, like OP, I would never have understood the joke by myself. The other answers show that an understanding of the joke requires some basic cultural assumptions - here, that nuns are secretly lewd (and sometimes, apparently, lesbian). When the one nun asks, 'Where's the soap?', the other takes this as a declarative comment about her current (apparently masturbatory) use of the soap: 'Wears [out] the soap.' The guilty nun then affirms that 'Yes, it does, rather.' Why do some people find this funny? Because they already have a history of finding sexualized nuns funny, and this joke triggers their comic affection for that idea. There are innumerable jokes that similarly necessitate some level of preconceived notion for their humor to be apparent. Let's look, for instance, at this 'dumb blonde' joke: The waiter asked the blonde if she would like her pizza cut into six pieces or twelve. 'Six, please,' she said. 'I could never eat twelve!' Why is this funny (to the extent that it is)? Because its audience has been programmed (probably from a young age) to find the idea that blondes are stupid funny. The joke itself doesn't much matter in its details - it just has to trigger that automatic comic response. Examples abound - jokes about how Jewish/Asian/[insert modifier here] men have small penises, jokes about how 'you might be a redneck if...', and on and on. Ultimately, they're contextual, and no amount of explaining the context will make the joke funny (even if it makes it decipherable).

PROBLEM: What goes in the blank? Commerce Commercial Finance Financial Trade __________.

SOLUTION:
One word is mercantile: Related to trade [ODO]