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