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