Ответ:

Решение:

  1. You shouldn't forget to do a good warm-up. (DONT)
    Don't forget to do a good warm-up.
  2. Start preparing a few months before the walk. (HAVE)
    You have to start preparing a few months before the walk.
  3. It's a good idea to practise with a friend. (SHOULD)
    You should practise with a friend.
  4. It's not necessary for you to buy special sports clothes (NOT HAVE TO)
    You do not have to buy special sports clothes.

Ответ:

1. Don't

2. have to

3. should

4. do not have to