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