I could yelp you could yelp he could yelp we could yelp you could yelp they could yelp
V-ing form
I could be yelping you could be yelping he could be yelping we could be yelping you could be yelping they could be yelping
Perfect
I could have yelped you could have yelped he could have yelped we could have yelped you could have yelped they could have yelped
V-ing perfect
I could have been yelping you could have been yelping he could have been yelping we could have been yelping you could have been yelping they could have been yelping
Negation
Simple form
I could not yelp you could not yelp he could not yelp we could not yelp you could not yelp they could not yelp
V-ing form
I could not be yelping you could not be yelping he could not be yelping we could not be yelping you could not be yelping they could not be yelping
Perfect
I could not have yelped you could not have yelped he could not have yelped we could not have yelped you could not have yelped they could not have yelped
V-ing perfect
I could not have been yelping you could not have been yelping he could not have been yelping we could not have been yelping you could not have been yelping they could not have been yelping
Interrogative form
Simple form
could I yelp? could you yelp? could he yelp? could we yelp? could you yelp? could they yelp?
V-ing form
could I be yelping? could you be yelping? could he be yelping? could we be yelping? could you be yelping? could they be yelping?
Perfect
could I have yelped? could you have yelped? could he have yelped? could we have yelped? could you have yelped? could they have yelped?
V-ing perfect
could I have been yelping? could you have been yelping? could he have been yelping? could we have been yelping? could you have been yelping? could they have been yelping?
Interro-negative form
Simple form
could I not yelp? could you not yelp? could he not yelp? could we not yelp? could you not yelp? could they not yelp?
V-ing form
could I not be yelping? could you not be yelping? could he not be yelping? could we not be yelping? could you not be yelping? could they not be yelping?
Perfect
could I not have yelped? could you not have yelped? could he not have yelped? could we not have yelped? could you not have yelped? could they not have yelped?
V-ing perfect
could I not have been yelping? could you not have been yelping? could he not have been yelping? could we not have been yelping? could you not have been yelping? could they not have been yelping?