instruction:
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.
question:
These two seem very similar. What are the major differences between the two? For example, in the following sentence, Substitute the lesser punishment for the greater one. Can one use 'less' instead of 'lesser' here?.
answer:
Less refers to quantity, lesser refers to quality. Your sentence could be rephrased as 'substitute less punishment for more punishment.' Here we are saying that the amount of punishment is smaller. But if you say 'substitute the lesser punishment' you are saying the type of punishment is not as severe. Forty lashes is less punishment than 100 lashes. Being pelted with rotten tomatoes is a lesser punishment than being pelted with rocks.


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.


question:
I'm talking about an incurable disease which is a lifelong affliction but is not fatal. Usage - She is suffering from a ____________ illness. I can't use terminal here because terminal means 'predicted to lead to death, especially slowly; incurable.'.
answer:

chronic From the Oxford English Dictionary (OED) Of diseases, etc.: Lasting a long time, long-continued, lingering, inveterate; opposed to acute OED cites as an example: 1994 Jrnl. Internal Med. 236 685 A 23-year-old female student exhibited all the characteristics of chronic factitious disorder with physical symptoms (Munchhausen's syndrome): deliberate simulation of illness, peregrination, fantastic pseudology and dramatic circumstances on admittance. (Emphasis added) OED contrasts chronic with acute acute: Of a disease, symptom, etc.: coming quickly to a crisis or conclusion; of rapid onset and short duration; of recent or sudden onset; contrasted with chronic The Cambridge English Dictionary gives as examples (rearranged): chronic arthritis/pain She suffers from chronic pain in her knees. I have a chronic bowel disease