I can warn you can warn he can warn we can warn you can warn they can warn
V-ing form
I can be warning you can be warning he can be warning we can be warning you can be warning they can be warning
Perfect
I can have warned you can have warned he can have warned we can have warned you can have warned they can have warned
V-ing perfect
I can have been warning you can have been warning he can have been warning we can have been warning you can have been warning they can have been warning
Negation
Simple form
I cannot warn you cannot warn he cannot warn we cannot warn you cannot warn they cannot warn
V-ing form
I cannot be warning you cannot be warning he cannot be warning we cannot be warning you cannot be warning they cannot be warning
Perfect
I cannot have warned you cannot have warned he cannot have warned we cannot have warned you cannot have warned they cannot have warned
V-ing perfect
I cannot have been warning you cannot have been warning he cannot have been warning we cannot have been warning you cannot have been warning they cannot have been warning
Interrogative form
Simple form
can I warn? can you warn? can he warn? can we warn? can you warn? can they warn?
V-ing form
can I be warning? can you be warning? can he be warning? can we be warning? can you be warning? can they be warning?
Perfect
can I have warned? can you have warned? can he have warned? can we have warned? can you have warned? can they have warned?
V-ing perfect
can I have been warning? can you have been warning? can he have been warning? can we have been warning? can you have been warning? can they have been warning?
Interro-negative form
Simple form
can I not warn? can you not warn? can he not warn? can we not warn? can you not warn? can they not warn?
V-ing form
can I not be warning? can you not be warning? can he not be warning? can we not be warning? can you not be warning? can they not be warning?
Perfect
can I not have warned? can you not have warned? can he not have warned? can we not have warned? can you not have warned? can they not have warned?
V-ing perfect
can I not have been warning? can you not have been warning? can he not have been warning? can we not have been warning? can you not have been warning? can they not have been warning?