Решение:
- Everyone must swim between the two red and yellow flags.
- You mustn't stay out of the water if the red flag is flying. You can't go swimming then because it's dangerous.
- You mustn't surf where other people are swimming. You must stay in the surfing area.
- You don't have to wear sun cream, but it's a good idea to wear it.
Ответ: must; mustn't, can't; mustn't, must; don't have to.