Why the word in the bracket include “in”? Enclosed with this letter is a pre-addressed, stamped envelope (in which) to mail your application for membership to our club. 1) Why the word in the bracket include “in”? 2) I understood that just “which” is impossible due to “to mail”, but why “in” if “of, on, with and so on” other preposition is impossible? 3) And how to make a sentence if I use other preposition such as “of, on, with”. Thanks

Please help (that, why) Martin’s job is demanding ( that, why ) he takes occsinal three-day leave to get away from the pressures of office work. 1) Which is better for branket? 2) And why? Please let me know example. Thanks!