I could stumble you could stumble he could stumble we could stumble you could stumble they could stumble
V-ing form
I could be stumbling you could be stumbling he could be stumbling we could be stumbling you could be stumbling they could be stumbling
Perfect
I could have stumbled you could have stumbled he could have stumbled we could have stumbled you could have stumbled they could have stumbled
V-ing perfect
I could have been stumbling you could have been stumbling he could have been stumbling we could have been stumbling you could have been stumbling they could have been stumbling
Negation
Simple form
I could not stumble you could not stumble he could not stumble we could not stumble you could not stumble they could not stumble
V-ing form
I could not be stumbling you could not be stumbling he could not be stumbling we could not be stumbling you could not be stumbling they could not be stumbling
Perfect
I could not have stumbled you could not have stumbled he could not have stumbled we could not have stumbled you could not have stumbled they could not have stumbled
V-ing perfect
I could not have been stumbling you could not have been stumbling he could not have been stumbling we could not have been stumbling you could not have been stumbling they could not have been stumbling
Interrogative form
Simple form
could I stumble? could you stumble? could he stumble? could we stumble? could you stumble? could they stumble?
V-ing form
could I be stumbling? could you be stumbling? could he be stumbling? could we be stumbling? could you be stumbling? could they be stumbling?
Perfect
could I have stumbled? could you have stumbled? could he have stumbled? could we have stumbled? could you have stumbled? could they have stumbled?
V-ing perfect
could I have been stumbling? could you have been stumbling? could he have been stumbling? could we have been stumbling? could you have been stumbling? could they have been stumbling?
Interro-negative form
Simple form
could I not stumble? could you not stumble? could he not stumble? could we not stumble? could you not stumble? could they not stumble?
V-ing form
could I not be stumbling? could you not be stumbling? could he not be stumbling? could we not be stumbling? could you not be stumbling? could they not be stumbling?
Perfect
could I not have stumbled? could you not have stumbled? could he not have stumbled? could we not have stumbled? could you not have stumbled? could they not have stumbled?
V-ing perfect
could I not have been stumbling? could you not have been stumbling? could he not have been stumbling? could we not have been stumbling? could you not have been stumbling? could they not have been stumbling?