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