I could excel you could excel she could excel we could excel you could excel they could excel
V-ing form
I could be excelling you could be excelling she could be excelling we could be excelling you could be excelling they could be excelling
Perfect
I could have excelled you could have excelled she could have excelled we could have excelled you could have excelled they could have excelled
V-ing perfect
I could have been excelling you could have been excelling she could have been excelling we could have been excelling you could have been excelling they could have been excelling
Negation
Simple form
I could not excel you could not excel she could not excel we could not excel you could not excel they could not excel
V-ing form
I could not be excelling you could not be excelling she could not be excelling we could not be excelling you could not be excelling they could not be excelling
Perfect
I could not have excelled you could not have excelled she could not have excelled we could not have excelled you could not have excelled they could not have excelled
V-ing perfect
I could not have been excelling you could not have been excelling she could not have been excelling we could not have been excelling you could not have been excelling they could not have been excelling
Interrogative form
Simple form
could I excel? could you excel? could she excel? could we excel? could you excel? could they excel?
V-ing form
could I be excelling? could you be excelling? could she be excelling? could we be excelling? could you be excelling? could they be excelling?
Perfect
could I have excelled? could you have excelled? could she have excelled? could we have excelled? could you have excelled? could they have excelled?
V-ing perfect
could I have been excelling? could you have been excelling? could she have been excelling? could we have been excelling? could you have been excelling? could they have been excelling?
Interro-negative form
Simple form
could I not excel? could you not excel? could she not excel? could we not excel? could you not excel? could they not excel?
V-ing form
could I not be excelling? could you not be excelling? could she not be excelling? could we not be excelling? could you not be excelling? could they not be excelling?
Perfect
could I not have excelled? could you not have excelled? could she not have excelled? could we not have excelled? could you not have excelled? could they not have excelled?
V-ing perfect
could I not have been excelling? could you not have been excelling? could she not have been excelling? could we not have been excelling? could you not have been excelling? could they not have been excelling?