I could decrease you could decrease he could decrease we could decrease you could decrease they could decrease
V-ing form
I could be decreasing you could be decreasing he could be decreasing we could be decreasing you could be decreasing they could be decreasing
Perfect
I could have decreased you could have decreased he could have decreased we could have decreased you could have decreased they could have decreased
V-ing perfect
I could have been decreasing you could have been decreasing he could have been decreasing we could have been decreasing you could have been decreasing they could have been decreasing
Negation
Simple form
I couldn't decrease you couldn't decrease he couldn't decrease we couldn't decrease you couldn't decrease they couldn't decrease
V-ing form
I couldn't be decreasing you couldn't be decreasing he couldn't be decreasing we couldn't be decreasing you couldn't be decreasing they couldn't be decreasing
Perfect
I couldn't have decreased you couldn't have decreased he couldn't have decreased we couldn't have decreased you couldn't have decreased they couldn't have decreased
V-ing perfect
I couldn't have been decreasing you couldn't have been decreasing he couldn't have been decreasing we couldn't have been decreasing you couldn't have been decreasing they couldn't have been decreasing
Interrogative form
Simple form
could I decrease? could you decrease? could he decrease? could we decrease? could you decrease? could they decrease?
V-ing form
could I be decreasing? could you be decreasing? could he be decreasing? could we be decreasing? could you be decreasing? could they be decreasing?
Perfect
could I have decreased? could you have decreased? could he have decreased? could we have decreased? could you have decreased? could they have decreased?
V-ing perfect
could I have been decreasing? could you have been decreasing? could he have been decreasing? could we have been decreasing? could you have been decreasing? could they have been decreasing?
Interro-negative form
Simple form
couldn't I decrease? couldn't you decrease? couldn't he decrease? couldn't we decrease? couldn't you decrease? couldn't they decrease?
V-ing form
couldn't I be decreasing? couldn't you be decreasing? couldn't he be decreasing? couldn't we be decreasing? couldn't you be decreasing? couldn't they be decreasing?
Perfect
couldn't I have decreased? couldn't you have decreased? couldn't he have decreased? couldn't we have decreased? couldn't you have decreased? couldn't they have decreased?
V-ing perfect
couldn't I have been decreasing? couldn't you have been decreasing? couldn't he have been decreasing? couldn't we have been decreasing? couldn't you have been decreasing? couldn't they have been decreasing?