I still have a few more doubts.

1) Thank you for being so quick in answering my letter.
2) I wish you all a nice weekend and I look forward to receiving your email.
(Can you help me express these two concepts).

3) Could you start a question with "in between which students is he sitting?"
Is it possible to start a question with "between" and put "in" at the end?
If yes, could you give me an example?
4) Her parents always try to fulfil her wishes.

1) Thank you for replying so soon.

2) OK

3) Could you start a question with "in between which students is he sitting?"
Is it possible to start a question with "between" and put "in" at the end?
If yes, could you give me an example?
Better is to simply omit "in" - it's superfluous!

4) Her parents always try to provide what she wants and needs.

Certainly! I'd be happy to help you with your doubts.

1) To express your gratitude for a quick response to your letter, you could say:
"Thank you for your prompt response to my letter."

2) To convey that you wish someone a nice weekend and are looking forward to receiving their email, you could say:
"I wish you all a wonderful weekend and I eagerly await your email."

3) Yes, it is possible to start a question with "in between which students is he sitting?" In this case, "in between" is used as a prepositional phrase to indicate the location. However, it is not common to put "in" at the end of a question like this.

4) The statement "Her parents always try to fulfill her wishes" implies that her parents make an effort to satisfy or meet her desires.