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