could diphthongize
Regular verb: diphthongize - diphthongized- diphthongized

Affirmation 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
I could diphthongize
you could diphthongize
she could diphthongize
we could diphthongize
you could diphthongize
they could diphthongize 	I could be diphthongizing
you could be diphthongizing
she could be diphthongizing
we could be diphthongizing
you could be diphthongizing
they could be diphthongizing	I could have diphthongized
you could have diphthongized
she could have diphthongized
we could have diphthongized
you could have diphthongized
they could have diphthongized	I could have been diphthongizing
you could have been diphthongizing
she could have been diphthongizing
we could have been diphthongizing
you could have been diphthongizing
they could have been diphthongizing	

Negation 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
I couldn't diphthongize
you couldn't diphthongize
she couldn't diphthongize
we couldn't diphthongize
you couldn't diphthongize
they couldn't diphthongize 	I couldn't be diphthongizing
you couldn't be diphthongizing
she couldn't be diphthongizing
we couldn't be diphthongizing
you couldn't be diphthongizing
they couldn't be diphthongizing	I couldn't have diphthongized
you couldn't have diphthongized
she couldn't have diphthongized
we couldn't have diphthongized
you couldn't have diphthongized
they couldn't have diphthongized	I couldn't have been diphthongizing
you couldn't have been diphthongizing
she couldn't have been diphthongizing
we couldn't have been diphthongizing
you couldn't have been diphthongizing
they couldn't have been diphthongizing	

Interrogative form 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
could I diphthongize?
could you diphthongize?
could she diphthongize?
could we diphthongize?
could you diphthongize?
could they diphthongize? 	could I be diphthongizing?
could you be diphthongizing?
could she be diphthongizing?
could we be diphthongizing?
could you be diphthongizing?
could they be diphthongizing? 	could I have diphthongized?
could you have diphthongized?
could she have diphthongized?
could we have diphthongized?
could you have diphthongized?
could they have diphthongized? 	could I have been diphthongizing?
could you have been diphthongizing?
could she have been diphthongizing?
could we have been diphthongizing?
could you have been diphthongizing?
could they have been diphthongizing? 	

Interro-negative form 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
couldn't I diphthongize?
couldn't you diphthongize?
couldn't she diphthongize?
couldn't we diphthongize?
couldn't you diphthongize?
couldn't they diphthongize? 	couldn't I be diphthongizing?
couldn't you be diphthongizing?
couldn't she be diphthongizing?
couldn't we be diphthongizing?
couldn't you be diphthongizing?
couldn't they be diphthongizing? 	couldn't I have diphthongized?
couldn't you have diphthongized?
couldn't she have diphthongized?
couldn't we have diphthongized?
couldn't you have diphthongized?
couldn't they have diphthongized? 	couldn't I have been diphthongizing?
couldn't you have been diphthongizing?
couldn't she have been diphthongizing?
couldn't we have been diphthongizing?
couldn't you have been diphthongizing?
couldn't they have been diphthongizing?