
Regular verb: rain - rained - rained
Simple form | V-ing form | 
| - - it must rain - - -  | - - it must be raining - - -  | 
Perfect | V-ing perfect | 
| - - it must have rained - - -  | - - it must have been raining - - -  | 
Simple form | V-ing form | 
| - - it mustn't rain - - -  | - - it mustn't be raining - - -  | 
Perfect | V-ing perfect | 
| - - it mustn't have rained - - -  | - - it mustn't have been raining - - -  | 
Simple form | V-ing form | 
| - - must it rain? - - -  | - - must it be raining? - - -  | 
Perfect | V-ing perfect | 
| - - must it have rained? - - -  | - - must it have been raining? - - -  | 
Simple form | V-ing form | 
| - - mustn't it rain? - - -  | - - mustn't it be raining? - - -  | 
Perfect | V-ing perfect | 
| - - mustn't it have rained? - - -  | - - mustn't it have been raining? - - -  |