could whiten
Regular verb: whiten - whitened- whitened

Affirmation 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
I could whiten
you could whiten
she could whiten
we could whiten
you could whiten
they could whiten 	I could be whitening
you could be whitening
she could be whitening
we could be whitening
you could be whitening
they could be whitening	I could have whitened
you could have whitened
she could have whitened
we could have whitened
you could have whitened
they could have whitened	I could have been whitening
you could have been whitening
she could have been whitening
we could have been whitening
you could have been whitening
they could have been whitening	

Negation 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
I could not whiten
you could not whiten
she could not whiten
we could not whiten
you could not whiten
they could not whiten 	I could not be whitening
you could not be whitening
she could not be whitening
we could not be whitening
you could not be whitening
they could not be whitening	I could not have whitened
you could not have whitened
she could not have whitened
we could not have whitened
you could not have whitened
they could not have whitened	I could not have been whitening
you could not have been whitening
she could not have been whitening
we could not have been whitening
you could not have been whitening
they could not have been whitening	

Interrogative form 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
could I whiten?
could you whiten?
could she whiten?
could we whiten?
could you whiten?
could they whiten? 	could I be whitening?
could you be whitening?
could she be whitening?
could we be whitening?
could you be whitening?
could they be whitening? 	could I have whitened?
could you have whitened?
could she have whitened?
could we have whitened?
could you have whitened?
could they have whitened? 	could I have been whitening?
could you have been whitening?
could she have been whitening?
could we have been whitening?
could you have been whitening?
could they have been whitening? 	

Interro-negative form 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
could I not whiten?
could you not whiten?
could she not whiten?
could we not whiten?
could you not whiten?
could they not whiten? 	could I not be whitening?
could you not be whitening?
could she not be whitening?
could we not be whitening?
could you not be whitening?
could they not be whitening? 	could I not have whitened?
could you not have whitened?
could she not have whitened?
could we not have whitened?
could you not have whitened?
could they not have whitened? 	could I not have been whitening?
could you not have been whitening?
could she not have been whitening?
could we not have been whitening?
could you not have been whitening?
could they not have been whitening?