
Regular verb: snow - snowed - snowed
| Simple form | V-ing form | 
| - - it could snow - - - | - - it could be snowing - - - | 
| Perfect | V-ing perfect | 
| - - it could have snowed - - - | - - it could have been snowing - - - | 
| Simple form | V-ing form | 
| - - it could not snow - - - | - - it could not be snowing - - - | 
| Perfect | V-ing perfect | 
| - - it could not have snowed - - - | - - it could not have been snowing - - - | 
| Simple form | V-ing form | 
| - - could it snow? - - - | - - could it be snowing? - - - | 
| Perfect | V-ing perfect | 
| - - could it have snowed? - - - | - - could it have been snowing? - - - | 
| Simple form | V-ing form | 
| - - could it not snow? - - - | - - could it not be snowing? - - - | 
| Perfect | V-ing perfect | 
| - - could it not have snowed? - - - | - - could it not have been snowing? - - - |