I could warn you could warn he could warn we could warn you could warn they could warn
V-ing form
I could be warning you could be warning he could be warning we could be warning you could be warning they could be warning
Perfect
I could have warned you could have warned he could have warned we could have warned you could have warned they could have warned
V-ing perfect
I could have been warning you could have been warning he could have been warning we could have been warning you could have been warning they could have been warning
Negation
Simple form
I couldn't warn you couldn't warn he couldn't warn we couldn't warn you couldn't warn they couldn't warn
V-ing form
I couldn't be warning you couldn't be warning he couldn't be warning we couldn't be warning you couldn't be warning they couldn't be warning
Perfect
I couldn't have warned you couldn't have warned he couldn't have warned we couldn't have warned you couldn't have warned they couldn't have warned
V-ing perfect
I couldn't have been warning you couldn't have been warning he couldn't have been warning we couldn't have been warning you couldn't have been warning they couldn't have been warning
Interrogative form
Simple form
could I warn? could you warn? could he warn? could we warn? could you warn? could they warn?
V-ing form
could I be warning? could you be warning? could he be warning? could we be warning? could you be warning? could they be warning?
Perfect
could I have warned? could you have warned? could he have warned? could we have warned? could you have warned? could they have warned?
V-ing perfect
could I have been warning? could you have been warning? could he have been warning? could we have been warning? could you have been warning? could they have been warning?
Interro-negative form
Simple form
couldn't I warn? couldn't you warn? couldn't he warn? couldn't we warn? couldn't you warn? couldn't they warn?
V-ing form
couldn't I be warning? couldn't you be warning? couldn't he be warning? couldn't we be warning? couldn't you be warning? couldn't they be warning?
Perfect
couldn't I have warned? couldn't you have warned? couldn't he have warned? couldn't we have warned? couldn't you have warned? couldn't they have warned?
V-ing perfect
couldn't I have been warning? couldn't you have been warning? couldn't he have been warning? couldn't we have been warning? couldn't you have been warning? couldn't they have been warning?