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