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