I could yawn you could yawn she could yawn we could yawn you could yawn they could yawn
V-ing form
I could be yawning you could be yawning she could be yawning we could be yawning you could be yawning they could be yawning
Perfect
I could have yawned you could have yawned she could have yawned we could have yawned you could have yawned they could have yawned
V-ing perfect
I could have been yawning you could have been yawning she could have been yawning we could have been yawning you could have been yawning they could have been yawning
Negation
Simple form
I could not yawn you could not yawn she could not yawn we could not yawn you could not yawn they could not yawn
V-ing form
I could not be yawning you could not be yawning she could not be yawning we could not be yawning you could not be yawning they could not be yawning
Perfect
I could not have yawned you could not have yawned she could not have yawned we could not have yawned you could not have yawned they could not have yawned
V-ing perfect
I could not have been yawning you could not have been yawning she could not have been yawning we could not have been yawning you could not have been yawning they could not have been yawning
Interrogative form
Simple form
could I yawn? could you yawn? could she yawn? could we yawn? could you yawn? could they yawn?
V-ing form
could I be yawning? could you be yawning? could she be yawning? could we be yawning? could you be yawning? could they be yawning?
Perfect
could I have yawned? could you have yawned? could she have yawned? could we have yawned? could you have yawned? could they have yawned?
V-ing perfect
could I have been yawning? could you have been yawning? could she have been yawning? could we have been yawning? could you have been yawning? could they have been yawning?
Interro-negative form
Simple form
could I not yawn? could you not yawn? could she not yawn? could we not yawn? could you not yawn? could they not yawn?
V-ing form
could I not be yawning? could you not be yawning? could she not be yawning? could we not be yawning? could you not be yawning? could they not be yawning?
Perfect
could I not have yawned? could you not have yawned? could she not have yawned? could we not have yawned? could you not have yawned? could they not have yawned?
V-ing perfect
could I not have been yawning? could you not have been yawning? could she not have been yawning? could we not have been yawning? could you not have been yawning? could they not have been yawning?