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I would like to know the difference between "ask" and "demand" and when to use each of them. - 

I would like to know the difference between BUY and PURCHASE and when to use each of them. --

Which of the following forms is more correct in English usage?

“What is the difference between "just" and "only?" How and when should I use them? Thank you" 

"What is the difference between "pandemic" and "epidemic?" They appear a lot in recent VOA news. Thanks a lot!" 

What is the difference between “please” and “kindly.” Some … use the two words together, e.g. “please kindly assist.”

Hi, I want to know the meaning of “through,” “throughout” and “go through.”

I am at a loss about how to use the following words correctly: score, mark, point, and grade. For example, if a student answers my question correctly in class, should I say, "I will give you two points for your success" or should I say, "I will give you two marks?" Another question: How should we call the numeral that we put on students' test papers? Is that a mark? Or grade? Or point? Or score? And can I say, "In this test, student A's grade is two points higher than student B’s"? Or should I use "mark" or maybe "score" instead of "grade" in this situation?

I've seen recently that we shouldn't call a teacher “Teacher." We should call him or her by their name. Is that true? Why? Is it the same for professors? 

Why is a sort of men’s underwear called “long johns”? Thank you.