I could strengthen you could strengthen he could strengthen we could strengthen you could strengthen they could strengthen
V-ing form
I could be strengthening you could be strengthening he could be strengthening we could be strengthening you could be strengthening they could be strengthening
Perfect
I could have strengthened you could have strengthened he could have strengthened we could have strengthened you could have strengthened they could have strengthened
V-ing perfect
I could have been strengthening you could have been strengthening he could have been strengthening we could have been strengthening you could have been strengthening they could have been strengthening
Negation
Simple form
I could not strengthen you could not strengthen he could not strengthen we could not strengthen you could not strengthen they could not strengthen
V-ing form
I could not be strengthening you could not be strengthening he could not be strengthening we could not be strengthening you could not be strengthening they could not be strengthening
Perfect
I could not have strengthened you could not have strengthened he could not have strengthened we could not have strengthened you could not have strengthened they could not have strengthened
V-ing perfect
I could not have been strengthening you could not have been strengthening he could not have been strengthening we could not have been strengthening you could not have been strengthening they could not have been strengthening
Interrogative form
Simple form
could I strengthen? could you strengthen? could he strengthen? could we strengthen? could you strengthen? could they strengthen?
V-ing form
could I be strengthening? could you be strengthening? could he be strengthening? could we be strengthening? could you be strengthening? could they be strengthening?
Perfect
could I have strengthened? could you have strengthened? could he have strengthened? could we have strengthened? could you have strengthened? could they have strengthened?
V-ing perfect
could I have been strengthening? could you have been strengthening? could he have been strengthening? could we have been strengthening? could you have been strengthening? could they have been strengthening?
Interro-negative form
Simple form
could I not strengthen? could you not strengthen? could he not strengthen? could we not strengthen? could you not strengthen? could they not strengthen?
V-ing form
could I not be strengthening? could you not be strengthening? could he not be strengthening? could we not be strengthening? could you not be strengthening? could they not be strengthening?
Perfect
could I not have strengthened? could you not have strengthened? could he not have strengthened? could we not have strengthened? could you not have strengthened? could they not have strengthened?
V-ing perfect
could I not have been strengthening? could you not have been strengthening? could he not have been strengthening? could we not have been strengthening? could you not have been strengthening? could they not have been strengthening?