English Language (ESL) Grammar Quizzes for Students - UsingEnglish.com

Reference to the link above, where answers have been given at the end of every quiz.
But I am unable to find the answer WHY of a quiz. Is there any page on this site which can explain the reasons as well?

For example,
Quiz: While & During
We met ___ the Xmas holidays.
where the answer is 'during'. But why the while? Explanation or logic?

Another example
Quiz: By or Until
I'll keep phoning ____ you pay me.

Where the answer is 'until'. But why? Logic?

Is there any page of explanation to these quizzes on this site?