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