I could master you could master she could master we could master you could master they could master
V-ing form
I could be mastering you could be mastering she could be mastering we could be mastering you could be mastering they could be mastering
Perfect
I could have mastered you could have mastered she 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 she 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 she 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 she 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 she 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 she 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 she master? could we master? could you master? could they master?
V-ing form
could I be mastering? could you be mastering? could she be mastering? could we be mastering? could you be mastering? could they be mastering?
Perfect
could I have mastered? could you have mastered? could she 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 she 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 she 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 she 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 she 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 she have been mastering? couldn't we have been mastering? couldn't you have been mastering? couldn't they have been mastering?