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