could substitute
Regular verb: substitute - substituted- substituted

Affirmation 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
I could substitute
you could substitute
she could substitute
we could substitute
you could substitute
they could substitute 	I could be substituting
you could be substituting
she could be substituting
we could be substituting
you could be substituting
they could be substituting	I could have substituted
you could have substituted
she could have substituted
we could have substituted
you could have substituted
they could have substituted	I could have been substituting
you could have been substituting
she could have been substituting
we could have been substituting
you could have been substituting
they could have been substituting	

Negation 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
I couldn't substitute
you couldn't substitute
she couldn't substitute
we couldn't substitute
you couldn't substitute
they couldn't substitute 	I couldn't be substituting
you couldn't be substituting
she couldn't be substituting
we couldn't be substituting
you couldn't be substituting
they couldn't be substituting	I couldn't have substituted
you couldn't have substituted
she couldn't have substituted
we couldn't have substituted
you couldn't have substituted
they couldn't have substituted	I couldn't have been substituting
you couldn't have been substituting
she couldn't have been substituting
we couldn't have been substituting
you couldn't have been substituting
they couldn't have been substituting	

Interrogative form 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
could I substitute?
could you substitute?
could she substitute?
could we substitute?
could you substitute?
could they substitute? 	could I be substituting?
could you be substituting?
could she be substituting?
could we be substituting?
could you be substituting?
could they be substituting? 	could I have substituted?
could you have substituted?
could she have substituted?
could we have substituted?
could you have substituted?
could they have substituted? 	could I have been substituting?
could you have been substituting?
could she have been substituting?
could we have been substituting?
could you have been substituting?
could they have been substituting? 	

Interro-negative form 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
couldn't I substitute?
couldn't you substitute?
couldn't she substitute?
couldn't we substitute?
couldn't you substitute?
couldn't they substitute? 	couldn't I be substituting?
couldn't you be substituting?
couldn't she be substituting?
couldn't we be substituting?
couldn't you be substituting?
couldn't they be substituting? 	couldn't I have substituted?
couldn't you have substituted?
couldn't she have substituted?
couldn't we have substituted?
couldn't you have substituted?
couldn't they have substituted? 	couldn't I have been substituting?
couldn't you have been substituting?
couldn't she have been substituting?
couldn't we have been substituting?
couldn't you have been substituting?
couldn't they have been substituting?