should happen
Regular verb: happen - happened- happened

Affirmation 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
-
-
it'd happen
-
-
- 	-
-
it'd be happening
-
-
- 	-
-
it should've happened
-
-
- 	-
-
it should've been happening
-
-
- 	

Negation 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
-
-
it shouldn't happen
-
-
- 	-
-
it shouldn't be happening
-
-
- 	-
-
it shouldn't have happened
-
-
- 	-
-
it shouldn't have been happening
-
-
- 	

Interrogative form 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
-
-
should it happen?
-
-
- 	-
-
should it be happening?
-
-
- 	-
-
should it have happened?
-
-
- 	-
-
should it have been happening?
-
-
- 	

Interro-negative form 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
-
-
shouldn't it happen?
-
-
- 	-
-
shouldn't it be happening?
-
-
- 	-
-
shouldn't it have happened?
-
-
- 	-
-
shouldn't it have been happening?
-
-
-