Regular verb: happen - happened - happened
Simple form | V-ing form |
- - it must happen - - - | - - it must be happening - - - |
Perfect | V-ing perfect |
- - it must have happened - - - | - - it must have been happening - - - |
Simple form | V-ing form |
- - it mustn't happen - - - | - - it mustn't be happening - - - |
Perfect | V-ing perfect |
- - it mustn't have happened - - - | - - it mustn't have been happening - - - |
Simple form | V-ing form |
- - must it happen? - - - | - - must it be happening? - - - |
Perfect | V-ing perfect |
- - must it have happened? - - - | - - must it have been happening? - - - |
Simple form | V-ing form |
- - mustn't it happen? - - - | - - mustn't it be happening? - - - |
Perfect | V-ing perfect |
- - mustn't it have happened? - - - | - - mustn't it have been happening? - - - |