I could whiten you could whiten she could whiten we could whiten you could whiten they could whiten
V-ing form
I could be whitening you could be whitening she could be whitening we could be whitening you could be whitening they could be whitening
Perfect
I could have whitened you could have whitened she could have whitened we could have whitened you could have whitened they could have whitened
V-ing perfect
I could have been whitening you could have been whitening she could have been whitening we could have been whitening you could have been whitening they could have been whitening
Negation
Simple form
I couldn't whiten you couldn't whiten she couldn't whiten we couldn't whiten you couldn't whiten they couldn't whiten
V-ing form
I couldn't be whitening you couldn't be whitening she couldn't be whitening we couldn't be whitening you couldn't be whitening they couldn't be whitening
Perfect
I couldn't have whitened you couldn't have whitened she couldn't have whitened we couldn't have whitened you couldn't have whitened they couldn't have whitened
V-ing perfect
I couldn't have been whitening you couldn't have been whitening she couldn't have been whitening we couldn't have been whitening you couldn't have been whitening they couldn't have been whitening
Interrogative form
Simple form
could I whiten? could you whiten? could she whiten? could we whiten? could you whiten? could they whiten?
V-ing form
could I be whitening? could you be whitening? could she be whitening? could we be whitening? could you be whitening? could they be whitening?
Perfect
could I have whitened? could you have whitened? could she have whitened? could we have whitened? could you have whitened? could they have whitened?
V-ing perfect
could I have been whitening? could you have been whitening? could she have been whitening? could we have been whitening? could you have been whitening? could they have been whitening?
Interro-negative form
Simple form
couldn't I whiten? couldn't you whiten? couldn't she whiten? couldn't we whiten? couldn't you whiten? couldn't they whiten?
V-ing form
couldn't I be whitening? couldn't you be whitening? couldn't she be whitening? couldn't we be whitening? couldn't you be whitening? couldn't they be whitening?
Perfect
couldn't I have whitened? couldn't you have whitened? couldn't she have whitened? couldn't we have whitened? couldn't you have whitened? couldn't they have whitened?
V-ing perfect
couldn't I have been whitening? couldn't you have been whitening? couldn't she have been whitening? couldn't we have been whitening? couldn't you have been whitening? couldn't they have been whitening?