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