It means, for example if I say, "I go to the park every now and then", then it isn't a common event, but happens often enough not to be classified as a rarity.
Answers:
1. Can’t be
2. Can’t have won
3. Must have been
4. Must be
5. Couldn’t have been
6. May have been
7. Ought to have left
8. Can’t be
9. Could turn out
10. Must have left
11. Should have won
12. Should be
he enjoys nature and wants someone to share it with
