I could whiten you could whiten he could whiten we could whiten you could whiten they could whiten
V-ing form
I could be whitening you could be whitening he could be whitening we could be whitening you could be whitening they could be whitening
Perfect
I could have whitened you could have whitened he 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 he 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 he 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 he 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 he 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 he 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 he whiten? could we whiten? could you whiten? could they whiten?
V-ing form
could I be whitening? could you be whitening? could he be whitening? could we be whitening? could you be whitening? could they be whitening?
Perfect
could I have whitened? could you have whitened? could he 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 he 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 he 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 he 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 he 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 he have been whitening? couldn't we have been whitening? couldn't you have been whitening? couldn't they have been whitening?