I can doubt you can doubt he can doubt we can doubt you can doubt they can doubt
V-ing form
I can be doubting you can be doubting he can be doubting we can be doubting you can be doubting they can be doubting
Perfect
I can have doubted you can have doubted he can have doubted we can have doubted you can have doubted they can have doubted
V-ing perfect
I can have been doubting you can have been doubting he can have been doubting we can have been doubting you can have been doubting they can have been doubting
Negation
Simple form
I can't doubt you can't doubt he can't doubt we can't doubt you can't doubt they can't doubt
V-ing form
I can't be doubting you can't be doubting he can't be doubting we can't be doubting you can't be doubting they can't be doubting
Perfect
I can't have doubted you can't have doubted he can't have doubted we can't have doubted you can't have doubted they can't have doubted
V-ing perfect
I can't have been doubting you can't have been doubting he can't have been doubting we can't have been doubting you can't have been doubting they can't have been doubting
Interrogative form
Simple form
can I doubt? can you doubt? can he doubt? can we doubt? can you doubt? can they doubt?
V-ing form
can I be doubting? can you be doubting? can he be doubting? can we be doubting? can you be doubting? can they be doubting?
Perfect
can I have doubted? can you have doubted? can he have doubted? can we have doubted? can you have doubted? can they have doubted?
V-ing perfect
can I have been doubting? can you have been doubting? can he have been doubting? can we have been doubting? can you have been doubting? can they have been doubting?
Interro-negative form
Simple form
can't I doubt? can't you doubt? can't he doubt? can't we doubt? can't you doubt? can't they doubt?
V-ing form
can't I be doubting? can't you be doubting? can't he be doubting? can't we be doubting? can't you be doubting? can't they be doubting?
Perfect
can't I have doubted? can't you have doubted? can't he have doubted? can't we have doubted? can't you have doubted? can't they have doubted?
V-ing perfect
can't I have been doubting? can't you have been doubting? can't he have been doubting? can't we have been doubting? can't you have been doubting? can't they have been doubting?