Teacher: 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.
Teacher: Now, understand the problem? If you are still confused, see the following example:
How are i.e. and e.g. pronounced?
Solution: i.e. stands for id est (Latin), which means 'that is'. You use it to link in a deeper explanation about something. Pronounce it 'eye - ee'. e.g. stands for exempli gratia (also Latin), which means 'for example'. You use it to link in an example of a more generic term. Pronounce it 'ee - jee'
Reason: Perfect answer because it's consistent with context uses required information from the question to provide the detailed answer with an example.

Now, solve this instance: Is singular correct, or is plural of 'credential' correct for a single username & password pair?.
Student:
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.