I could tumble you could tumble he could tumble we could tumble you could tumble they could tumble
V-ing form
I could be tumbling you could be tumbling he could be tumbling we could be tumbling you could be tumbling they could be tumbling
Perfect
I could have tumbled you could have tumbled he could have tumbled we could have tumbled you could have tumbled they could have tumbled
V-ing perfect
I could have been tumbling you could have been tumbling he could have been tumbling we could have been tumbling you could have been tumbling they could have been tumbling
Negation
Simple form
I could not tumble you could not tumble he could not tumble we could not tumble you could not tumble they could not tumble
V-ing form
I could not be tumbling you could not be tumbling he could not be tumbling we could not be tumbling you could not be tumbling they could not be tumbling
Perfect
I could not have tumbled you could not have tumbled he could not have tumbled we could not have tumbled you could not have tumbled they could not have tumbled
V-ing perfect
I could not have been tumbling you could not have been tumbling he could not have been tumbling we could not have been tumbling you could not have been tumbling they could not have been tumbling
Interrogative form
Simple form
could I tumble? could you tumble? could he tumble? could we tumble? could you tumble? could they tumble?
V-ing form
could I be tumbling? could you be tumbling? could he be tumbling? could we be tumbling? could you be tumbling? could they be tumbling?
Perfect
could I have tumbled? could you have tumbled? could he have tumbled? could we have tumbled? could you have tumbled? could they have tumbled?
V-ing perfect
could I have been tumbling? could you have been tumbling? could he have been tumbling? could we have been tumbling? could you have been tumbling? could they have been tumbling?
Interro-negative form
Simple form
could I not tumble? could you not tumble? could he not tumble? could we not tumble? could you not tumble? could they not tumble?
V-ing form
could I not be tumbling? could you not be tumbling? could he not be tumbling? could we not be tumbling? could you not be tumbling? could they not be tumbling?
Perfect
could I not have tumbled? could you not have tumbled? could he not have tumbled? could we not have tumbled? could you not have tumbled? could they not have tumbled?
V-ing perfect
could I not have been tumbling? could you not have been tumbling? could he not have been tumbling? could we not have been tumbling? could you not have been tumbling? could they not have been tumbling?