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