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: Coming from a technical background I'm slightly confused. What is the difference between simultaneously and concurrently? How do we use these words?.

SOLUTION: I just looked these up in an online dictionary; this is what I found: concurrently: overlapping in duration simultaneously: at the same instant Both of them mean 'at the same time,' and are almost interchangeable. That said, if I had to point out a slight difference, I'd say that concurrently occurs over a longer time, and is a bit less synchronized than simultaneously. For example: The two processors were running concurrently, then they crashed simultaneously. In that example, I wouldn't swap the two words in question.

PROBLEM: Possible Duplicate: Why don't we use the indefinite article with 'software'? In France I have always been told that saying 'a software' is not correct English (as a nominal compound), and that 'a piece of software' or 'a software program/package/product/system' must be used instead. Recently I have doubts... is there any case where it is actually correct? Examples found on the Internet: SalsaJ, a software for data analysis at school So if you sold a software which required your customer to pay a monthly fee, [...].

SOLUTION:
No, this is always wrong. Both examples you provide contain incorrect usage of the term 'software.' (A mistake is still a mistake even if many people make it.) However, there might be special cases where you'd see the article preceding software. For example: A software solution would be better for the problem than a hardware one. The indefinite article 'a' modifies 'solution', not 'software', in this case in spite of preceding 'software' in this case.