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 couldn't strengthen you couldn't strengthen he couldn't strengthen we couldn't strengthen you couldn't strengthen they couldn't strengthen
V-ing form
I couldn't be strengthening you couldn't be strengthening he couldn't be strengthening we couldn't be strengthening you couldn't be strengthening they couldn't be strengthening
Perfect
I couldn't have strengthened you couldn't have strengthened he couldn't have strengthened we couldn't have strengthened you couldn't have strengthened they couldn't have strengthened
V-ing perfect
I couldn't have been strengthening you couldn't have been strengthening he couldn't have been strengthening we couldn't have been strengthening you couldn't have been strengthening they couldn't 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
couldn't I strengthen? couldn't you strengthen? couldn't he strengthen? couldn't we strengthen? couldn't you strengthen? couldn't they strengthen?
V-ing form
couldn't I be strengthening? couldn't you be strengthening? couldn't he be strengthening? couldn't we be strengthening? couldn't you be strengthening? couldn't they be strengthening?
Perfect
couldn't I have strengthened? couldn't you have strengthened? couldn't he have strengthened? couldn't we have strengthened? couldn't you have strengthened? couldn't they have strengthened?
V-ing perfect
couldn't I have been strengthening? couldn't you have been strengthening? couldn't he have been strengthening? couldn't we have been strengthening? couldn't you have been strengthening? couldn't they have been strengthening?