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