English verb conjugation
COULD STIFF

Regular verb: stiff - stiffed - stiffed

Affirmation

Simple form	V-ing form
I could stiff you could stiff he could stiff we could stiff you could stiff they could stiff	I could be stiffing you could be stiffing he could be stiffing we could be stiffing you could be stiffing they could be stiffing
Perfect	V-ing perfect
I could have stiffed you could have stiffed he could have stiffed we could have stiffed you could have stiffed they could have stiffed	I could have been stiffing you could have been stiffing he could have been stiffing we could have been stiffing you could have been stiffing they could have been stiffing

Negation

Simple form	V-ing form
I couldn't stiff you couldn't stiff he couldn't stiff we couldn't stiff you couldn't stiff they couldn't stiff	I couldn't be stiffing you couldn't be stiffing he couldn't be stiffing we couldn't be stiffing you couldn't be stiffing they couldn't be stiffing
Perfect	V-ing perfect
I couldn't have stiffed you couldn't have stiffed he couldn't have stiffed we couldn't have stiffed you couldn't have stiffed they couldn't have stiffed	I couldn't have been stiffing you couldn't have been stiffing he couldn't have been stiffing we couldn't have been stiffing you couldn't have been stiffing they couldn't have been stiffing

Interrogative form

Simple form	V-ing form
could I stiff? could you stiff? could he stiff? could we stiff? could you stiff? could they stiff?	could I be stiffing? could you be stiffing? could he be stiffing? could we be stiffing? could you be stiffing? could they be stiffing?
Perfect	V-ing perfect
could I have stiffed? could you have stiffed? could he have stiffed? could we have stiffed? could you have stiffed? could they have stiffed?	could I have been stiffing? could you have been stiffing? could he have been stiffing? could we have been stiffing? could you have been stiffing? could they have been stiffing?

Interro-negative form

Simple form	V-ing form
couldn't I stiff? couldn't you stiff? couldn't he stiff? couldn't we stiff? couldn't you stiff? couldn't they stiff?	couldn't I be stiffing? couldn't you be stiffing? couldn't he be stiffing? couldn't we be stiffing? couldn't you be stiffing? couldn't they be stiffing?
Perfect	V-ing perfect
couldn't I have stiffed? couldn't you have stiffed? couldn't he have stiffed? couldn't we have stiffed? couldn't you have stiffed? couldn't they have stiffed?	couldn't I have been stiffing? couldn't you have been stiffing? couldn't he have been stiffing? couldn't we have been stiffing? couldn't you have been stiffing? couldn't they have been stiffing?