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