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