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