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