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 must not happen - - - | - - it must not be happening - - - |
Perfect | V-ing perfect |
- - it must not have happened - - - | - - it must not 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 |
- - must it not happen? - - - | - - must it not be happening? - - - |
Perfect | V-ing perfect |
- - must it not have happened? - - - | - - must it not have been happening? - - - |