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