English verb conjugation
CAN HAPPEN

Regular verb: happen - happened - happened

Affirmation

Simple form	V-ing form
- - it can happen - - -	- - it can be happening - - -
Perfect	V-ing perfect
- - it can have happened - - -	- - it can have been happening - - -

Negation

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

Interrogative form

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

Interro-negative form

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