Could you please clarify these doubts for me? Thank you.

Question

1.He admitted to eating all the biscuits. (I s “to” necessary??)
2.I’m worried about Brian’s arriving so late. (about Brian arriving so late/ that Brian arrives so late). Are the two forms in parenthesis possible?
To start/begin with, I’ll have potato soup (a potato soup, a soup of potatoes) Are the two forms in brackets possible?
3.I couldn’t help postponing the meeting (I couldn’t do anything but postpone .../I could do nothing except postpone the meeting). ) Are the two forms in brackets possible?
What are you doing this evening? (What are you going to do this evening? Possible?)
4.I’m staying at home . I’m going to write some letters. (I’m writing some letters is wrong)

Answer 1

1. With or without "to" is fine.

2. Wording in parentheses is incorrect; since "arriving" is a gerund here, there needs to be a possessive in front of it.
http://grammar.ccc.commnet.edu/grammar/gerunds.htm#possessive

Wording in parentheses is also incorrect about potato soup!

3. Wording in parentheses is OK -- wordy, but OK.

4. Wording in parentheses is OK.