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