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